MINOR: log: add a new field "%lc" to implement a per-frontend log counter

Sometimes it would be convenient to have a log counter so that from a log
server we know whether some logs were lost or not. The frontend's log counter
serves exactly this purpose. It's incremented each time a traffic log is
produced. If a log is disabled using "http-request set-log-level silent",
the counter will not be incremented. However, admin logs are not accounted
for. Also, if logs are filtered out before being sent to the server because
of a minimum level set on the log line, the counter will be increased anyway.

The counter is 32-bit, so it will wrap, but that's not an issue considering
that 4 billion logs are rarely in the same file, let alone close to each
other.
diff --git a/include/types/log.h b/include/types/log.h
index c7e47ea..c680663 100644
--- a/include/types/log.h
+++ b/include/types/log.h
@@ -53,6 +53,7 @@
 	LOG_FMT_SERVERPORT,
 	LOG_FMT_SERVERIP,
 	LOG_FMT_COUNTER,
+	LOG_FMT_LOGCNT,
 	LOG_FMT_PID,
 	LOG_FMT_DATE,
 	LOG_FMT_DATEGMT,
diff --git a/include/types/proxy.h b/include/types/proxy.h
index b31df74..748f4aa 100644
--- a/include/types/proxy.h
+++ b/include/types/proxy.h
@@ -316,6 +316,8 @@
 	int (*accept)(struct session *s);       /* application layer's accept() */
 	struct conn_src conn_src;               /* connection source settings */
 	struct proxy *next;
+
+	unsigned int log_count;			/* number of logs produced by the frontend */
 	struct list logsrvs;
 	struct list logformat; 			/* log_format linked list */
 	char *header_unique_id; 		/* unique-id header */