Add dynamic default query context

[jtuglu1]

Description

Query context can currently be supplied via 3 places:

1. The per-query context override field.
2. The static node configuration provided at Druid process boot.
3. The query context defaults in the code itself.

Neither of these two options are helpful for tuning "default" query context values as with 1) you need to constantly re-deploy as you tune and 2) you rely on the user to set these values (not good).

This introduces a fourth query context resolution layer in the broker dynamic config. Resolution order is now:

1. The per-query context
2. The broker dynamic config value
3. The static node configuration value
4. The default values in the code itself

Release note

Add dynamic default query context to brokers. Allows operators to quickly tune the default setting for query context values without requiring a re-deployment or forcing upstream clients to update their queries.

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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.

[gianm]

Useful idea.

[gianm]

For the blocking we have this warning:

Note: Query blocking is best-effort. Queries may not be blocked in certain cases, such as when a Broker has recently started and hasn't received the config yet, or if the Broker cannot contact the Coordinator. Brokers poll the configuration periodically (default every 1 minute) and also receive push updates from the Coordinator for immediate propagation.

I think we need a similar warning for context. Maybe put this warning at the top of the "Broker dynamic configuration" section and make clear it applies to everything in here. I say this because in case default query context properties are "important" (i.e. operators really want them to always be set) they may be surprised that they are in fact not necessarily always going to be set.

This also makes me think at some point we might want to add a mode to make them actually guaranteed, i.e., block Broker startup until first sync of dynamic config. Doesn't have to be done in this PR.

[jtuglu1]

Sure. I think we can make this an opt-in config on the broker, but I think it should be a separate change.

[jtuglu1]

@gianm thoughts here?

[jtuglu1]

@gianm any thoughts here?