[DOC][MINOR] Add metrics instance 'mesos_cluster' to monitoring doc

## What changes were proposed in this pull request?

Metrics instance "mesos_cluster" exists in spark, but not mentioned in monitoring.md. This PR add it.

## How was this patch tested?

Manually test.

Closes apache#23691 from SongYadong/doc_mesos_metrics_inst.

Authored-by: SongYadong <[email protected]>
Signed-off-by: Sean Owen <[email protected]>

Author: SongYadong

docs/monitoring.md

@@ -659,6 +659,7 @@ set of sinks to which metrics are reported. The following instances are currentl
 * `driver`: The Spark driver process (the process in which your SparkContext is created).
 * `shuffleService`: The Spark shuffle service.
 * `applicationMaster`: The Spark ApplicationMaster when running on YARN.
+* `mesos_cluster`: The Spark cluster scheduler when running on Mesos.
 
 Each instance can report to zero or more _sinks_. Sinks are contained in the
 `org.apache.spark.metrics.sink` package: