MEDIUM: counters: add a new "gpc0_rate" counter in stick-tables

This counter is special in that instead of reporting the gpc0 cumulative
count, it returns its increase rate over the configured period.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index a031cde..f9d7558 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -6085,6 +6085,13 @@
       to put a special tag on some entries, for instance to note that a
       specific behaviour was detected and must be known for future matches.
 
+    - gpc0_rate(<period>) : increment rate of the first General Purpose Counter
+      over a period. It is a positive 32-bit integer integer which may be used
+      for anything. Just like <gpc0>, it counts events, but instead of keeping
+      a cumulative count, it maintains the rate at which the counter is
+      incremented. Most of the time it will be used to measure the frequency of
+      occurrence of certain events (eg: requests to a specific URL).
+
     - conn_cnt : Connection Count. It is a positive 32-bit integer which counts
       the absolute number of connections received from clients which matched
       this entry. It does not mean the connections were accepted, just that
@@ -8527,7 +8534,17 @@
 sc2_get_gpc0 <integer>
 sc3_get_gpc0 <integer>
   Returns the value of the first General Purpose Counter associated to the
-  currently tracked counters. See also src_get_gpc0 and sc1/sc2_inc_gpc0.
+  currently tracked counters. See also src_get_gpc0 and sc1/sc2/sc3_get_gpc0.
+
+sc1_gpc0_rate <integer>
+sc2_gpc0_rate <integer>
+sc3_gpc0_rate <integer>
+  Returns the average increment rate of the first General Purpose Counter
+  associated to the currently tracked counters. It reports the frequency
+  which the gpc0 counter was incremented over the configured period. See also
+  src_gpc0_rate, sc1/sc2/sc3_get_gpc0, and sc1/sc2/sc3_inc_gpc0. Note that the
+  "gpc0_rate" counter must be stored in the stick-table for a value to be
+  returned, as "gpc0" only holds the event count.
 
 sc1_http_err_cnt <integer>
 sc2_http_err_cnt <integer>
@@ -8684,6 +8701,16 @@
   the designated stick-table. If the address is not found, zero is returned.
   See also sc1/sc2_get_gpc0 and src_inc_gpc0.
 
+src_gpc0_rate <integer>
+src_gpc0_rate(<table>) <integer>
+  Returns the average increment rate of the first General Purpose Counter
+  associated to the connection's source IPv4 address in the current proxy's
+  stick-table or in the designated stick-table. It reports the frequency
+  which the gpc0 counter was incremented over the configured period. See also
+  sc1/sc2/sc3_gpc0_rate, src_get_gpc0, and sc1/sc2/sc3_inc_gpc0. Note that the
+  "gpc0_rate" counter must be stored in the stick-table for a value to be
+  returned, as "gpc0" only holds the event count.
+
 src_http_err_cnt <integer>
 src_http_err_cnt(<table>) <integer>
   Returns the cumulated number of HTTP errors from the current connection's
@@ -10149,6 +10176,17 @@
                associated to the currently tracked counters. See also
                src_get_gpc0 and sc1/sc2_inc_gpc0.
 
+  sc1_gpc0_rate
+  sc2_gpc0_rate
+  sc3_gpc0_rate
+               Returns the average increment rate of the first General Purpose
+               Counter associated to the currently tracked counters. It reports
+               the frequency which the gpc0 counter was incremented over the
+               configured period. See also src_gpc0_rate, sc1/sc2/sc3_get_gpc0,
+               and sc1/sc2/sc3_inc_gpc0. Note that the "gpc0_rate" counter must
+               be stored in the stick-table for a value to be returned, as
+               "gpc0" only holds the event count.
+
   sc1_http_err_cnt
   sc2_http_err_cnt
   sc3_http_err_cnt
@@ -10326,6 +10364,16 @@
                the address is not found, zero is returned.  See also
                sc1/sc2_get_gpc0 and src_inc_gpc0.
 
+  src_gpc0_rate([<table>])
+               Returns the average increment rate of the first General Purpose
+               Counter associated to the connection's source IPv4 address in
+               the current proxy's stick-table or in the designated stick-
+               table. It reports the frequency which the gpc0 counter was
+               incremented over the configured period. See also src_gpc0_rate,
+               sc1/sc2/sc3_get_gpc0, and sc1/sc2/sc3_inc_gpc0. Note that the
+               "gpc0_rate" counter must be stored in the stick-table for a
+               value to be returned, as "gpc0" only holds the event count.
+
   src_http_err_cnt([<table>])
                Returns the cumulated number of HTTP errors from the current
                connection's source IPv4 address in the current proxy's
diff --git a/include/types/stick_table.h b/include/types/stick_table.h
index 520f57a..dcdc405 100644
--- a/include/types/stick_table.h
+++ b/include/types/stick_table.h
@@ -46,6 +46,7 @@
 enum {
 	STKTABLE_DT_SERVER_ID,    /* the server ID to use with this session if > 0 */
 	STKTABLE_DT_GPC0,         /* General Purpose Counter 0 (unsigned 32-bit integer) */
+	STKTABLE_DT_GPC0_RATE,    /* General Purpose Counter 0's event rate */
 	STKTABLE_DT_CONN_CNT,     /* cumulated number of connections */
 	STKTABLE_DT_CONN_RATE,    /* incoming connection rate */
 	STKTABLE_DT_CONN_CUR,     /* concurrent number of connections */
@@ -88,6 +89,7 @@
 	/* types of each storable data */
 	int server_id;
 	unsigned int gpc0;
+	struct freq_ctr_period gpc0_rate;
 	unsigned int conn_cnt;
 	struct freq_ctr_period conn_rate;
 	unsigned int conn_cur;
diff --git a/src/session.c b/src/session.c
index 10d3feb..7437fe8 100644
--- a/src/session.c
+++ b/src/session.c
@@ -2645,6 +2645,77 @@
 	return smp_fetch_get_gpc0(&px->table, smp, stktable_lookup_key(&px->table, key));
 }
 
+/* set temp integer to the General Purpose Counter 0's event rate in the stksess entry <ts> */
+static int
+smp_fetch_gpc0_rate(struct stktable *table, struct sample *smp, struct stksess *ts)
+{
+	smp->flags = SMP_F_VOL_TEST;
+	smp->type = SMP_T_UINT;
+	smp->data.uint = 0;
+	if (ts != NULL) {
+		void *ptr = stktable_data_ptr(table, ts, STKTABLE_DT_GPC0_RATE);
+		if (!ptr)
+			return 0; /* parameter not stored */
+		smp->data.uint = read_freq_ctr_period(&stktable_data_cast(ptr, gpc0_rate),
+					       table->data_arg[STKTABLE_DT_GPC0_RATE].u);
+	}
+	return 1;
+}
+
+/* set temp integer to the General Purpose Counter 0's event rate from the
+ * session's tracked frontend counters.
+ */
+static int
+smp_fetch_sc1_gpc0_rate(struct proxy *px, struct session *l4, void *l7, unsigned int opt,
+                       const struct arg *args, struct sample *smp)
+{
+	if (!l4->stkctr[0].entry)
+		return 0;
+	return smp_fetch_gpc0_rate(l4->stkctr[0].table, smp, l4->stkctr[0].entry);
+}
+
+/* set temp integer to the General Purpose Counter 0's event rate from the
+ * session's tracked backend counters.
+ */
+static int
+smp_fetch_sc2_gpc0_rate(struct proxy *px, struct session *l4, void *l7, unsigned int opt,
+                       const struct arg *args, struct sample *smp)
+{
+	if (!l4->stkctr[1].entry)
+		return 0;
+	return smp_fetch_gpc0_rate(l4->stkctr[1].table, smp, l4->stkctr[1].entry);
+}
+
+/* set temp integer to the General Purpose Counter 0's event rate from the
+ * session's tracked backend counters.
+ */
+static int
+smp_fetch_sc3_gpc0_rate(struct proxy *px, struct session *l4, void *l7, unsigned int opt,
+                       const struct arg *args, struct sample *smp)
+{
+	if (!l4->stkctr[2].entry)
+		return 0;
+	return smp_fetch_gpc0_rate(l4->stkctr[2].table, smp, l4->stkctr[2].entry);
+}
+
+/* set temp integer to the General Purpose Counter 0's event rate from the
+ * session's source address in the table pointed to by expr.
+ * Accepts exactly 1 argument of type table.
+ */
+static int
+smp_fetch_src_gpc0_rate(struct proxy *px, struct session *l4, void *l7, unsigned int opt,
+                       const struct arg *args, struct sample *smp)
+{
+	struct stktable_key *key;
+
+	key = addr_to_stktable_key(&l4->si[0].conn->addr.from);
+	if (!key)
+		return 0;
+
+	px = args->data.prx;
+	return smp_fetch_gpc0_rate(&px->table, smp, stktable_lookup_key(&px->table, key));
+}
+
 /* Increment the General Purpose Counter 0 value in the stksess entry <ts> and
  * return it into temp integer.
  */
@@ -2655,10 +2726,22 @@
 	smp->type = SMP_T_UINT;
 	smp->data.uint = 0;
 	if (ts != NULL) {
-		void *ptr = stktable_data_ptr(table, ts, STKTABLE_DT_GPC0);
-		if (!ptr)
-			return 0; /* parameter not stored */
-		smp->data.uint = ++stktable_data_cast(ptr, gpc0);
+		void *ptr;
+
+		/* First, update gpc0_rate if it's tracked. Second, update its
+		 * gpc0 if tracked. Returns gpc0's value otherwise the curr_ctr.
+		 */
+		ptr = stktable_data_ptr(table, ts, STKTABLE_DT_GPC0_RATE);
+		if (ptr) {
+			update_freq_ctr_period(&stktable_data_cast(ptr, gpc0_rate),
+					       table->data_arg[STKTABLE_DT_GPC0_RATE].u, 1);
+			smp->data.uint = (&stktable_data_cast(ptr, gpc0_rate))->curr_ctr;
+		}
+
+		ptr = stktable_data_ptr(table, ts, STKTABLE_DT_GPC0);
+		if (ptr)
+			smp->data.uint = ++stktable_data_cast(ptr, gpc0);
+
 	}
 	return 1;
 }
@@ -3825,6 +3908,7 @@
 	{ "sc1_conn_cur",       NULL, acl_parse_int, acl_match_int },
 	{ "sc1_conn_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc1_get_gpc0",       NULL, acl_parse_int, acl_match_int },
+	{ "sc1_gpc0_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc1_http_err_cnt",   NULL, acl_parse_int, acl_match_int },
 	{ "sc1_http_err_rate",  NULL, acl_parse_int, acl_match_int },
 	{ "sc1_http_req_cnt",   NULL, acl_parse_int, acl_match_int },
@@ -3842,6 +3926,7 @@
 	{ "sc2_conn_cur",       NULL, acl_parse_int, acl_match_int },
 	{ "sc2_conn_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc2_get_gpc0",       NULL, acl_parse_int, acl_match_int },
+	{ "sc2_gpc0_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc2_http_err_cnt",   NULL, acl_parse_int, acl_match_int },
 	{ "sc2_http_err_rate",  NULL, acl_parse_int, acl_match_int },
 	{ "sc2_http_req_cnt",   NULL, acl_parse_int, acl_match_int },
@@ -3859,6 +3944,7 @@
 	{ "sc3_conn_cur",       NULL, acl_parse_int, acl_match_int },
 	{ "sc3_conn_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc3_get_gpc0",       NULL, acl_parse_int, acl_match_int },
+	{ "sc3_gpc0_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "sc3_http_err_cnt",   NULL, acl_parse_int, acl_match_int },
 	{ "sc3_http_err_rate",  NULL, acl_parse_int, acl_match_int },
 	{ "sc3_http_req_cnt",   NULL, acl_parse_int, acl_match_int },
@@ -3876,6 +3962,7 @@
 	{ "src_conn_cur",       NULL, acl_parse_int, acl_match_int },
 	{ "src_conn_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "src_get_gpc0",       NULL, acl_parse_int, acl_match_int },
+	{ "src_gpc0_rate",      NULL, acl_parse_int, acl_match_int },
 	{ "src_http_err_cnt",   NULL, acl_parse_int, acl_match_int },
 	{ "src_http_err_rate",  NULL, acl_parse_int, acl_match_int },
 	{ "src_http_req_cnt",   NULL, acl_parse_int, acl_match_int },
@@ -3902,6 +3989,7 @@
 	{ "sc1_conn_cur",       smp_fetch_sc1_conn_cur,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc1_conn_rate",      smp_fetch_sc1_conn_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc1_get_gpc0",       smp_fetch_sc1_get_gpc0,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
+	{ "sc1_gpc0_rate",      smp_fetch_sc1_gpc0_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc1_http_err_cnt",   smp_fetch_sc1_http_err_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc1_http_err_rate",  smp_fetch_sc1_http_err_rate,  0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc1_http_req_cnt",   smp_fetch_sc1_http_req_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
@@ -3919,6 +4007,7 @@
 	{ "sc2_conn_cur",       smp_fetch_sc2_conn_cur,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc2_conn_rate",      smp_fetch_sc2_conn_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc2_get_gpc0",       smp_fetch_sc2_get_gpc0,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
+	{ "sc2_gpc0_rate",      smp_fetch_sc2_gpc0_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc2_http_err_cnt",   smp_fetch_sc2_http_err_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc2_http_err_rate",  smp_fetch_sc2_http_err_rate,  0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc2_http_req_cnt",   smp_fetch_sc2_http_req_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
@@ -3936,6 +4025,7 @@
 	{ "sc3_conn_cur",       smp_fetch_sc3_conn_cur,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc3_conn_rate",      smp_fetch_sc3_conn_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc3_get_gpc0",       smp_fetch_sc3_get_gpc0,       0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
+	{ "sc3_gpc0_rate",      smp_fetch_sc3_gpc0_rate,      0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc3_http_err_cnt",   smp_fetch_sc3_http_err_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc3_http_err_rate",  smp_fetch_sc3_http_err_rate,  0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
 	{ "sc3_http_req_cnt",   smp_fetch_sc3_http_req_cnt,   0,           NULL, SMP_T_UINT, SMP_USE_INTRN, },
@@ -3953,6 +4043,7 @@
 	{ "src_conn_cur",       smp_fetch_src_conn_cur,       ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
 	{ "src_conn_rate",      smp_fetch_src_conn_rate,      ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
 	{ "src_get_gpc0",       smp_fetch_src_get_gpc0,       ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
+	{ "src_gpc0_rate",      smp_fetch_src_gpc0_rate,      ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
 	{ "src_http_err_cnt",   smp_fetch_src_http_err_cnt,   ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
 	{ "src_http_err_rate",  smp_fetch_src_http_err_rate,  ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
 	{ "src_http_req_cnt",   smp_fetch_src_http_req_cnt,   ARG1(1,TAB), NULL, SMP_T_UINT, SMP_USE_L4CLI, },
diff --git a/src/stick_table.c b/src/stick_table.c
index 3097e66..c357b32 100644
--- a/src/stick_table.c
+++ b/src/stick_table.c
@@ -685,6 +685,7 @@
 struct stktable_data_type stktable_data_types[STKTABLE_DATA_TYPES] = {
 	[STKTABLE_DT_SERVER_ID]     = { .name = "server_id",      .std_type = STD_T_SINT  },
 	[STKTABLE_DT_GPC0]          = { .name = "gpc0",           .std_type = STD_T_UINT  },
+	[STKTABLE_DT_GPC0_RATE]     = { .name = "gpc0_rate",      .std_type = STD_T_FRQP, .arg_type = ARG_T_DELAY  },
 	[STKTABLE_DT_CONN_CNT]      = { .name = "conn_cnt",       .std_type = STD_T_UINT  },
 	[STKTABLE_DT_CONN_RATE]     = { .name = "conn_rate",      .std_type = STD_T_FRQP, .arg_type = ARG_T_DELAY  },
 	[STKTABLE_DT_CONN_CUR]      = { .name = "conn_cur",       .std_type = STD_T_UINT  },