[7.13] add explain api endpoint test (#98060)

[alvarezmelissa87]

Backports the following commits to 7.13:

• add explain api endpoint test ([ML] API Integration tests: adds test for Data Frame Analytics explain endpoint #98060)

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[kibanamachine]

💛 Build succeeded, but was flaky

• continuous-integration/kibana-ci/pull-request

• Commit: 09e4216

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• Flaky suites:

  • xpack-kibana-ciGroup6

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Test Failures

Kibana Pipeline / general / X-Pack API Integration Tests.x-pack/test/api_integration/apis/searchprofiler/searchprofiler·ts.apis Search Profiler Profile should return profile results for a valid index

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Standard Out

Failed Tests Reporter:
  - Test has failed 3 times on tracked branches: https://github.com/elastic/kibana/issues/97954

[00:00:00]       │
[00:00:00]         └-: apis
[00:00:00]           └-> "before all" hook in "apis"
[00:18:07]           └-: Search Profiler
[00:18:07]             └-> "before all" hook in "Search Profiler"
[00:18:07]             └-: Profile
[00:18:07]               └-> "before all" hook for "should return profile results for a valid index"
[00:18:07]               └-> should return profile results for a valid index
[00:18:07]                 └-> "before each" hook: global before each for "should return profile results for a valid index"
[00:18:14]                 │ info [o.e.c.m.MetadataMappingService] [kibana-ci-immutable-centos-tests-xxl-1619188335204008107] [.kibana_7.13.0_001/cCnUHascRs2YqTbMdYqukw] update_mapping [_doc]
[00:18:26]                 └- ✖ fail: apis Search Profiler Profile should return profile results for a valid index
[00:18:26]                 │       Error: Maximumresponse size reached
[00:18:26]                 │       at Stream.<anonymous> (/dev/shm/workspace/parallel/3/kibana/node_modules/supertest/node_modules/superagent/lib/node/index.js:875:23)
[00:18:26]                 │       at Unzip.<anonymous> (/dev/shm/workspace/parallel/3/kibana/node_modules/supertest/node_modules/superagent/lib/node/unzip.js:48:30)
[00:18:26]                 │       at addChunk (internal/streams/readable.js:309:12)
[00:18:26]                 │       at readableAddChunk (internal/streams/readable.js:284:9)
[00:18:26]                 │       at Unzip.Readable.push (internal/streams/readable.js:223:10)
[00:18:26]                 │       at Unzip.Transform.push (internal/streams/transform.js:166:32)
[00:18:26]                 │       at Zlib.processCallback (zlib.js:539:10)
[00:18:26]                 │ 
[00:18:26]                 │ 

Stack Trace

Error: Maximum response size reached
    at Stream.<anonymous> (/dev/shm/workspace/parallel/3/kibana/node_modules/supertest/node_modules/superagent/lib/node/index.js:875:23)
    at Unzip.<anonymous> (/dev/shm/workspace/parallel/3/kibana/node_modules/supertest/node_modules/superagent/lib/node/unzip.js:48:30)
    at addChunk (internal/streams/readable.js:309:12)
    at readableAddChunk (internal/streams/readable.js:284:9)
    at Unzip.Readable.push (internal/streams/readable.js:223:10)
    at Unzip.Transform.push (internal/streams/transform.js:166:32)
    at Zlib.processCallback (zlib.js:539:10) {
  code: 'ETOOLARGE',
  response: undefined
}

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Metrics [docs]

✅ unchanged

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