Add support for value_count for exponential_histogram field

[JonasKunz]

Part of #135625.

Adds support of the exponential_histogram for the value_count queryDSL query.
The implementation only loads the corresponding doc value and avoids loading the entire histogram.

This code was again heavily inspired from the corresponding implementation for aggregate_metric_double

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)

[felixbarny]

Seems straightforward and follows what we're doing for aggregate metric double, both in terms of implementation and testing.

[felixbarny]

I think I've seen a variant of this several times. Maybe extract to a common helper method? That could be in the exponential histogram lib test module and we can import the test module in the aggregations module like this:

elasticsearch/x-pack/plugin/otel-data/build.gradle

Line 68 in ad0967a

testImplementation(testArtifact(project(":libs:exponential-histogram")))

Feel free to do this in a separate PR, whatever works best for you.

[JonasKunz]

Good point, there have been many variations of this. I'll try to put this in a lib in a separate PR

[martijnvg]

One nit and comment about yaml test, LGTM otherwise.

[martijnvg]

This should skip everything before 9.3.0? Also do these yaml tests run in mixed cluster qa?

[JonasKunz]

The yaml tests are currently started with this class:
https://github.com/elastic/elasticsearch/blob/main/x-pack/plugin/mapper-exponential-histogram/src/yamlRestTest/java/org/elasticsearch/xpack/exponentialhistogram/ExponentialHistogramYamlTestSuiteIT.java

It skips testing when running on snapshot builds, which now in hindsight seems like the wrong approach.
I already had a chat with @not-napoleon about this and have the following refactor on my todo list:

• Add a test feature in MapperFeature for exponential histograms, which is present/absent based on the feature flag
• Move the yaml tests into the shared x-pack yaml tests and make them require the feature above

This should ensure that the tests work properly with e.g. mixed cluster qa and also when we remove the feature flag eventually.

I'll do this refactor after my current pile of queryDSL-aggs branches was reduced, otherwise it introduces a lot of conflicts for now. We'll be forced to do it when removing the feature flag (otherwise tests will start to fail)m so we can't forget about it.