Need more metrics on minion task status

[mcvsubbu]

It will be useful to have the following metrics out of Pinot:

• The number of tasks and sub-tasks currently "in flight" (either in progress or in queue)
• The min/max/average/p95 etc. of a sub-task queue time
• The min/max/average/p95 etc. of sub-task execution time

[mcvsubbu]

PR #7091

[mcvsubbu]

PR #7099

[mayankshriv]

Thanks for adding this feature. We also wanted to add rest-api's to make these status queriable in a debug env. Hopefully we can reuse the code from #7099 when adding the rest-api.