Clarify compactTask/availableSlot/count metric description (#18663)

Clarify the compactTask/availableSlot/count metric description.

Author: abhishekrb19

docs/operations/metrics.md

@@ -413,7 +413,7 @@ These metrics are emitted by the Druid Coordinator in every run of the correspon
 |`tier/total/capacity`|Total capacity in bytes available in each tier.|`tier`|Varies|
 |`compact/task/count`|Number of tasks issued in the auto compaction run.| |Varies|
 |`compactTask/maxSlot/count`|Maximum number of task slots available for auto compaction tasks in the auto compaction run.| |Varies|
-|`compactTask/availableSlot/count`|Number of available task slots that can be used for auto compaction tasks in the auto compaction run. This is the max number of task slots minus any currently running compaction tasks.| |Varies|
+|`compactTask/availableSlot/count`|Number of currently vacant task slots out of the total slots allocated for auto compaction tasks. This value is computed as the difference between the total number of task slots allocated for auto compaction and the estimated number of task slots currently occupied by running compaction tasks. The number of sub-tasks of each compaction task is estimated to be `maxNumConcurrentSubTasks`.| |Varies|
 |`killTask/availableSlot/count`| Number of available task slots that can be used for auto kill tasks in the auto kill run. This is the max number of task slots minus any currently running auto kill tasks.                                                                                                                                                                                                                                                                                                                     | |Varies|
 |`killTask/maxSlot/count`| Maximum number of task slots available for auto kill tasks in the auto kill run.                                                                                                                                                                                                                                                                                                                                                                                                                | |Varies|
 |`kill/task/count`| Number of tasks issued in the auto kill run.                                                                                                                                                                                                                                                                                                                                                                                                                                                    | |Varies|