Track and emit segment loading rate for HttpLoadQueuePeon on Coordinator

[kfaraz]

Changes

• Add LoadingRateTracker which computes a moving average load rate based on the last few GBs of successful segment loads.
• Emit metric segment/loading/rateKbps from the Coordinator. In the future, we may also consider emitting this metric from the historicals themselves.
• Add expectedLoadTimeMillis to response of API /druid/coordinator/v1/loadQueue?simple

Testing done

• Added new unit tests
• Performed testing on local cluster with ~1M segments

Release note

• Emit new metric segment/loading/rateKbps from the Coordinator which tracks the current segment loading rate of a server. Dimensions: server
• Add expectedLoadTimeMillis to response of API /druid/coordinator/v1/loadQueue?simple

Future work

The expectedLoadTimeMillis can be used to show expected time on the web-console.

This PR does not include web-console changes and the screenshot below is only for reference.

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This PR has:

• [x] been self-reviewed.
  • [ ] using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• [x] added documentation for new or modified features or behaviors.
• [x] a release note entry in the PR description.
• [x] 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
• [x] added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• [x] added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• [ ] added integration tests.
• [x] been tested in a test Druid cluster.