client/finality-grandpa: Add Prometheus metrics to GossipValidator (#5237)

* client/finality-grandpa: Add Prometheus metrics to GossipValidator

Instrument finality grandpa `GossipValidator` exposing count of messages
validated by message type and message action.

```
\# HELP substrate_finality_grandpa_communication_gossip_validator_messages Number of messages validated by the finality grandpa gossip validator.
\# TYPE substrate_finality_grandpa_communication_gossip_validator_messages counter
substrate_finality_grandpa_communication_gossip_validator_messages{action="discard",message="neighbor"} 39
substrate_finality_grandpa_communication_gossip_validator_messages{action="keep",message="vote"} 28
```

* client/finality-grandpa: Add None as Prometheus registry in tests

* client/finality-granpda/src/communication: Refactor metric registration

substrate/client/finality-grandpa/src/observer.rs

@@ -178,6 +178,7 @@ where
 		network,
 		config.clone(),
 		persistent_data.set_state.clone(),
+		None,
 	);
 
 	let observer_work = ObserverWork::new(