[MINOR] stats: report the number of requests intercepted by the frontend

These requests are mainly monitor requests, as well as stats requests when
the stats are processed by the frontend. Having this counter helps explain
the difference in number of sessions that is sometimes observed between a
frontend and a backend.
diff --git a/include/types/counters.h b/include/types/counters.h
index e3e051f..391915a 100644
--- a/include/types/counters.h
+++ b/include/types/counters.h
@@ -48,6 +48,7 @@
 	long long srv_aborts;                   /* aborted responses during DATA phase caused by the server */
 	long long retries;                      /* retried and redispatched connections (BE only) */
 	long long redispatches;                 /* retried and redispatched connections (BE only) */
+	long long intercepted_req;              /* number of monitoring or stats requests intercepted by the frontend */
 
 	union {
 		struct {