[LI-FIXUP] Enhance metrics tag for BrokerToControllerRequestManagers (

#454)

This should be a direct fix-up to the commit
- e37eaa0 [LI-HOTFIX] Add observability into AlterIsrManager (#453)

EXIT_CRITERIA = When the corresponding commit is dropped

Using this tag, the `kafka-server` can generate inGraph metrics for both `DefaultAlterIsrManager` and `TransferLeaderManager` (or even future broker-to-controller channel) together with one regex query
```
"kafka.server:type=*,name=*,requestManagerCategory=BrokerToControllerRequestManager"
```

core/src/main/scala/kafka/server/BrokerToControllerRequestManager.scala

@@ -65,7 +65,7 @@ abstract class AbstractBrokerToControllerRequestManager[Item <: BrokerToControll
   private val inflightRequest: AtomicBoolean = new AtomicBoolean(false)
 
   // Additional observability into the internal states
-  private val baseMetricTags = Map("class" -> getClass.getName)
+  private val baseMetricTags = Map("requestManagerCategory" -> BrokerToControllerRequestManager.getClass.getSimpleName.replaceAll("\\$$", ""))
   @volatile private var lastInflightRequestLockTimeMs = time.milliseconds()
   @VisibleForTesting private[server] val unsentItemQueueSizeGauge = newGauge("unsentItemQueueSize", () => unsentItemQueue.size, baseMetricTags)
   @VisibleForTesting private[server] val inflightRequestGauge = newGauge("inflightRequest", () => if (inflightRequest.get()) 1 else 0, baseMetricTags)