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/rpc/src/system/mod.rs

@@ -20,8 +20,9 @@
 mod tests;
 
 use futures::{future::BoxFuture, FutureExt, TryFutureExt};
-use futures::{channel::{mpsc, oneshot}, compat::Compat};
+use futures::{channel::oneshot, compat::Compat};
 use sc_rpc_api::Receiver;
+use sp_utils::mpsc::TracingUnboundedSender;
 use sp_runtime::traits::{self, Header as HeaderT};
 
 use self::error::Result;
@@ -33,7 +34,7 @@ pub use self::gen_client::Client as SystemClient;
 /// System API implementation
 pub struct System<B: traits::Block> {
 	info: SystemInfo,
-	send_back: mpsc::UnboundedSender<Request<B>>,
+	send_back: TracingUnboundedSender<Request<B>>,
 }
 
 /// Request to be processed.
@@ -59,7 +60,7 @@ impl<B: traits::Block> System<B> {
 	/// reading from that channel and answering the requests.
 	pub fn new(
 		info: SystemInfo,
-		send_back: mpsc::UnboundedSender<Request<B>>,
+		send_back: TracingUnboundedSender<Request<B>>,
 	) -> Self {
 		System {
 			info,