Additional Metrics collected and exposed via prometheus (#5414)

This PR refactors the metrics measuring and Prometheus exposing entity in sc-service into its own submodule and extends the parameters it exposes by:

- system load average (over one, five and 15min)
- the TCP connection state of the process (lsof), refs #5304
- number of tokio threads
- number of known forks
- counter for items in each unbounded queue (with internal unbounded channels)
- number of file descriptors opened by this process (*nix only at this point)
- number of system threads (*nix only at this point)

refs #4679

Co-authored-by: Max Inden <mail@max-inden.de>
Co-authored-by: Ashley <ashley.ruglys@gmail.com>

substrate/client/authority-discovery/src/tests.rs

@@ -119,6 +119,7 @@ impl<Block: BlockT> HeaderBackend<Block> for TestApi {
 			finalized_hash: Default::default(),
 			finalized_number: Zero::zero(),
 			genesis_hash: Default::default(),
+			number_leaves: Default::default(),
 		}
 	}