Track gossip subscriptions as a metric (#1445)

## Issue Addressed
#1399 

## Proposed Changes
Set an Int gauge per topic and inc/dec when peers subscribe/unsubscribe

beacon_node/eth2_libp2p/src/metrics.rs

@@ -34,6 +34,11 @@ lazy_static! {
         "Unsolicited discovery requests per ip per second",
         &["Addresses"]
     );
+    pub static ref GOSSIPSUB_SUBSCRIBED_PEERS_COUNT: Result<IntGaugeVec> = try_create_int_gauge_vec(
+        "gossipsub_peers_per_topic_count",
+        "Peers subscribed per topic",
+        &["topic_hash"]
+    );
 }
 
 pub fn scrape_discovery_metrics() {