[Alerting/Monitoring] Add an API to get the server shutDownInProgress config and expose the info at tenant level.

Can we add an API to get the server shutDownInProgress config and expose the info at tenant level?

We recently saw that some upsert servers can become unresponsive and stop serving any queries after a server restart. Even the server is healthy, the shutDownInProgress flag can not be set back to false (ready to serve queries). This caused an incident on our side and it took us some time to dig into the broker logs to find out that some servers became unresponsive for a long time.

It would be useful if we can add an API to get server shutDownInProgress config and expose the info at tenant level so that we can proper alerting to avoid such incidents again.

related pr: https://github.com/apache/pinot/pull/8525

Start by reviewing related PR #8525 and tracing how the server shutDownInProgress flag is reported for upsert servers. Define the tenant-level API and its alerting-visible output, then verify that it identifies servers that are healthy but not ready to serve queries.

Written by the indexing model from the issue text.