Add prometheus registry to transaction pool, with couple of initial metrics (#5657)

* make new contructor

* add metrics to txpool

* fix review

* fix doc comment

* change to counters

* Update client/transaction-pool/src/metrics.rs

Co-Authored-By: Max Inden <mail@max-inden.de>

* Update client/transaction-pool/src/metrics.rs

Co-Authored-By: Max Inden <mail@max-inden.de>

* Update client/transaction-pool/src/metrics.rs

Co-Authored-By: Max Inden <mail@max-inden.de>

* Update client/transaction-pool/src/lib.rs

Co-Authored-By: Max Inden <mail@max-inden.de>

* Update client/transaction-pool/src/lib.rs

Co-Authored-By: Max Inden <mail@max-inden.de>

* use dedicated wrapper

Co-authored-by: Max Inden <mail@max-inden.de>

substrate/client/rpc/src/author/tests.rs

@@ -65,6 +65,7 @@ impl Default for TestSetup {
 		let pool = Arc::new(BasicPool::new(
 			Default::default(),
 			Arc::new(FullChainApi::new(client.clone())),
+			None,
 		).0);
 		TestSetup {
 			runtime: runtime::Runtime::new().expect("Failed to create runtime in test setup"),