Add task start status to worker report

[adarshsanjeev]

Adds two new fields to the worker report, pendingTasks and runningTasks.

These indicate the number of tasks (including controller) that are in pending and and have already fully started. These can be used in the UI to better communicate the state of a task. Since the controller running is a prerequisite for the worker report, the running task number will be at least 1.

New payload:

{
  "multiStageQuery": {
    "taskId": "query-37b4ee2e-3469-41c8-a74a-faec7b555980",
    "payload": {
      "status": {
        "status": "RUNNING",
        "startTime": "2022-10-25T08:22:52.925Z",
        "durationMs": 545,
        "pendingTasks": 2,
        "runningTasks": 2
      },
      "stages": [ <unchanged> ],
      "counters": {}
}

---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[cryptoe]

Thanks for the PR @adarshsanjeev .
Minor comments, LGTM overall. Please add the new payload in the PR description.
Will approve once the minor comments are looked into.

[LakshSingla]

Thanks for the PR! Left some minor comments on the same

[LakshSingla]

Should this change be documented somewhere? Perhaps in the release notes if there's no appropriate section in the documentation?

[vogievetsky]

I think 2 doc lines should be added here: https://github.com/apache/druid/blob/master/docs/multi-stage-query/api.md#L555

[LakshSingla]

Code style question, would it be clearer if instead of a Pair, we create a simple class (in the task launcher itself) that encapsulates these numbers? I am fine either way since this is used only once, but it would be clearer to read if someone is just going through the ControllerImpl class.
PR LGTM apart from this. Thanks!

[adarshsanjeev]

I did consider that, but since it is only used in a single place, I used a Pair, to avoid creating a class for a single call.

[cryptoe]

LGTM. Thanks @adarshsanjeev !!