[DISCUSS] Dropping the task audit log table from metastore

Druid currently has a table to store task audit logs in metastore. Every taskAction is stored in this table if they need to be audited (https://github.com/druid-io/druid/blob/master/indexing-service/src/main/java/io/druid/indexing/common/actions/TaskAction.java#L47). Basically, all taskActions modifying metastore like `SegmentInsertAction` currently need to be audited. If a taskAction needs to be audited, it is serialized into a byte array and stored as a blob in metastore.

In Druid, there is a single use case for the task audit log table: an overlord API to get a list of segments generated by a task ([http://:/druid/indexer/v1/task/{taskId}/segments](http://druid.io/docs/latest/design/indexing-service.html#submitting-tasks-and-querying-task-status)). This API needs to find all `SegmentInsertAction`s and `SegmentTransactionalInsertAction`s performed by the given task. AFAIK, this API isn't used widely. Outside of Druid, AFAIK, there is no good use case for task audit logs table.

The problem is that this table can cause an issue when inserting a large taskAction. For example, a `SegmentInsertAction` can be large if there are many segments to be inserted in that action. This can cause some issues. For example, in MySQL, it can cause the insertion error if the data size is larger than [max_allowed_packet](https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_max_allowed_packet).

I tried to mitigate this in https://github.com/druid-io/druid/pull/5751, but looks not a good idea. Another suggestion in that PR was to store audit logs in deep storage, but it requires quite a lot of changes and introduces new design issues. I don't think we need to make a large effort to fix such an issue that has no good use case.

So, I'm suggesting dropping the task audit log table. Here is my suggestion.

1) Disabling task audit logging by default in 0.13. We need to add a configuration for optionally enabling task audit logging. The overlord API should also be deprecated.
2) Completely removing the task audit log table and the overlord API in 0.14.

Any thoughts?

Start by reading indexing-service/src/main/java/io/druid/indexing/common/actions/TaskAction.java and the existing task audit-log implementation, then review PR 5751. Trace the task/{taskId}/segments Overlord API and its SegmentInsertAction and SegmentTransactionalInsertAction use cases. Done would require a decided migration and removal or configuration plan for the table and API.

Written by the indexing model from the issue text.