Monitor transactions rejected from the pool as invalid (#5992)

substrate/client/transaction-pool/src/metrics.rs

@@ -45,6 +45,7 @@ impl MetricsLink {
 pub struct Metrics {
 	pub validations_scheduled: Counter<U64>,
 	pub validations_finished: Counter<U64>,
+	pub validations_invalid: Counter<U64>,
 }
 
 impl Metrics {
@@ -64,6 +65,13 @@ impl Metrics {
 				)?,
 				registry,
 			)?,
+			validations_invalid: register(
+				Counter::new(
+					"sub_txpool_validations_invalid",
+					"Total number of transactions that were removed from the pool as invalid",
+				)?,
+				registry,
+			)?,
 		})
 	}
 }