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/src/leaves.rs

@@ -195,6 +195,11 @@ impl<H, N> LeafSet<H, N> where
 		self.storage.iter().flat_map(|(_, hashes)| hashes.iter()).cloned().collect()
 	}
 
+	/// Number of known leaves
+	pub fn count(&self) -> usize {
+		self.storage.len()
+	}
+
 	/// Write the leaf list to the database transaction.
 	pub fn prepare_transaction(&mut self, tx: &mut DBTransaction, column: u32, prefix: &[u8]) {
 		let mut buf = prefix.to_vec();