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/communication/mod.rs

@@ -30,6 +30,7 @@
 use futures::{prelude::*, channel::mpsc};
 use log::{debug, trace};
 use parking_lot::Mutex;
+use prometheus_endpoint::Registry;
 use std::{pin::Pin, sync::Arc, task::{Context, Poll}};
 
 use finality_grandpa::Message::{Prevote, Precommit, PrimaryPropose};
@@ -178,10 +179,12 @@ impl<B: BlockT, N: Network<B>> NetworkBridge<B, N> {
 		service: N,
 		config: crate::Config,
 		set_state: crate::environment::SharedVoterSetState<B>,
+		prometheus_registry: Option<&Registry>,
 	) -> Self {
 		let (validator, report_stream) = GossipValidator::new(
 			config,
 			set_state.clone(),
+			prometheus_registry,
 		);
 
 		let validator = Arc::new(validator);