Add transaction pool size metric (#1869)

Adds `besu_transaction_pool_transactions` metric reporting the size
of the transaction pool.

Signed-off-by: Danno Ferrin <danno.ferrin@gmail.com>

CHANGELOG.md

@@ -1,8 +1,9 @@
 # Changelog
 
-## 21.1.0-RC2
+## 21.1.1
 
 ### Additions and Improvements
+* Added `besu_transaction_pool_transactions` to the reported metrics, counting the mempool size [\#1869](https://github.com/hyperledger/besu/pull/1869)
 
 ### Bug Fixes
 

ethereum/eth/src/main/java/org/hyperledger/besu/ethereum/eth/transactions/PendingTransactions.java

@@ -123,6 +123,12 @@ public class PendingTransactions {
             "Count of transactions removed from the transaction pool",
             "source",
             "operation");
+
+    metricsSystem.createIntegerGauge(
+        BesuMetricCategory.TRANSACTION_POOL,
+        "transactions",
+        "Current size of the transaction pool",
+        pendingTransactions::size);
   }
 
   public void evictOldTransactions() {