protocols/gossipsub: Add mesh metrics (#2316)

Enable instrumenting mesh through metrics and add gossipsub to
misc/metrics.

Co-authored-by: Max Inden <mail@max-inden.de>

protocols/gossipsub/src/lib.rs

@@ -132,6 +132,7 @@ mod gossip_promises;
 mod handler;
 mod interval;
 mod mcache;
+pub mod metrics;
 mod peer_score;
 pub mod subscription_filter;
 pub mod time_cache;