CCS test coverage for index filtering on ES|QL field_caps call

[astefan]

Description

Following the implementation of field_caps retry mechanism in ES|QL using an index filter, test coverage is needed on CCS-related scenarios.

[elasticsearchmachine]

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

[elasticsearchmachine]

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

[smalyshev]

Implemented by #119619

[astefan]

I am reopening this one because, while the code in #119619 is great and covers a lot of cases, I believe there are some use cases that need to be checked:

• is the ccs metadata (when asked for) still correctly returned in all cases where filtering is also used (ccs metadata when clusters are available/unavailable)? Maybe this can be added to the same RequestIndexingFilteringIT that executes all the tests in RequestIndexFilteringTestCase.
• some good tests are also in CrossClustersQueryIT; some time and research is needed to see which of these are good scenarios to test with index filtering as well.

[smalyshev]

@astefan I'm not sure we have now tests in RequestIndexFilteringIT when any clusters are unavailable. It uses only one remote cluster there, so there's not much place for that. There are other multi-cluster tests (e.g. in CrossClustersQueryIT) but they don't test filters.

I can certainly start with adding metadata checks in RequestIndexFilteringIT. Then I could see if anything else needs to be covered.

[astefan]

Thank you @smalyshev. Adding metadata checking to RequestIndexFilteringIT is ok with me. This test class and checks you added with this PR test scenarios where everything is ok from cluster availability point of view with queries that use filters.

In my vision, the same tests from RequestIndexFilteringIT but in the setup used by CrossClusterQueryIT still need to be added. This basically should cover clusters metadata accuracy when filters are used and either one or two _field_caps calls are performed in a CCS scenario where clusters are unavailable.