MINOR: stats: add the estimated need of concurrent connections per server

The max_used_conns value is used as an estimate of the needed number of
connections on a server to know how many to keep open. But this one is
not reported, making it hard to troubleshoot reuse issues. Let's export
it in the sessions/current column.
diff --git a/include/haproxy/stats-t.h b/include/haproxy/stats-t.h
index c5d68dd..01c399d 100644
--- a/include/haproxy/stats-t.h
+++ b/include/haproxy/stats-t.h
@@ -426,6 +426,7 @@
 	ST_F_IDLE_CONN_CUR,
 	ST_F_SAFE_CONN_CUR,
 	ST_F_USED_CONN_CUR,
+	ST_F_NEED_CONN_EST,
 
 	/* must always be the last one */
 	ST_F_TOTAL_FIELDS
diff --git a/src/stats.c b/src/stats.c
index 351b79f..1a453c3 100644
--- a/src/stats.c
+++ b/src/stats.c
@@ -247,6 +247,7 @@
 	[ST_F_IDLE_CONN_CUR]                 = { .name = "idle_conn_cur",               .desc = "Current number of unsafe idle connections"},
 	[ST_F_SAFE_CONN_CUR]                 = { .name = "safe_conn_cur",               .desc = "Current number of safe idle connections"},
 	[ST_F_USED_CONN_CUR]                 = { .name = "used_conn_cur",               .desc = "Current number of connections in use"},
+	[ST_F_NEED_CONN_EST]                 = { .name = "need_conn_est",               .desc = "Estimated needed number of connections"},
 };
 
 /* one line of info */
@@ -997,6 +998,7 @@
 		                "<tr><th>Current idle connections:</th><td>%s</td></tr>"
 		                "<tr><th>- unsafe:</th><td>%s</td></tr>"
 		                "<tr><th>- safe:</th><td>%s</td></tr>"
+		                "<tr><th>Estimated need of connections:</th><td>%s</td></tr>"
 		                "<tr><th>Active connections limit:</th><td>%s</td></tr>"
 		                "<tr><th>Idle connections limit:</th><td>%s</td></tr>"
 			        "</table></div></u>"
@@ -1010,6 +1012,7 @@
 			      U2H(stats[ST_F_SRV_ICUR].u.u32),
 			      U2H(stats[ST_F_IDLE_CONN_CUR].u.u32),
 			      U2H(stats[ST_F_SAFE_CONN_CUR].u.u32),
+			      U2H(stats[ST_F_NEED_CONN_EST].u.u32),
 
 			        LIM2A(stats[ST_F_SLIM].u.u32, "-"),
 		                stats[ST_F_SRV_ILIM].type ? U2H(stats[ST_F_SRV_ILIM].u.u32) : "-",
@@ -1703,6 +1706,7 @@
 	stats[ST_F_IDLE_CONN_CUR] = mkf_u32(0, sv->curr_idle_nb);
 	stats[ST_F_SAFE_CONN_CUR] = mkf_u32(0, sv->curr_safe_nb);
 	stats[ST_F_USED_CONN_CUR] = mkf_u32(0, sv->curr_used_conns);
+	stats[ST_F_NEED_CONN_EST] = mkf_u32(0, sv->est_need_conns);
 
 	/* status */
 	fld_status = chunk_newstr(out);