Update Documentation on meregeBuffer/pendingRequests for Real-time no…

…des (apache#16992)

apache#15025 adds mergeBuffer/pendingRequests metric in QueryCountStatsMonitor. Since real-time nodes also use the same merge buffers for queries and have QueryCountStatsMonitor , the documentation is being updated to include this metric.

docs/operations/metrics.md

@@ -119,6 +119,7 @@ Most metric values reset each emission period, as specified in `druid.monitoring
 |`query/failed/count`|Number of failed queries.|This metric is only available if the `QueryCountStatsMonitor` module is included.||
 |`query/interrupted/count`|Number of queries interrupted due to cancellation.|This metric is only available if the `QueryCountStatsMonitor` module is included.||
 |`query/timeout/count`|Number of timed out queries.|This metric is only available if the `QueryCountStatsMonitor` module is included.||
+|`mergeBuffer/pendingRequests`|Number of requests waiting to acquire a batch of buffers from the merge buffer pool.|This metric is only available if the `QueryCountStatsMonitor` module is included.||
 
 ### Jetty