[test] Tail based sampling (GA)

[simitt]

In order to make tail based sampling GA the feature requires some exhaustive testing.

This should include, but is not limited to:

• test with at least one service using an actual apm agent
• test that logs are sufficient and helpful (on error, when reaching max number of dynamic services etc. )
• test on a larger scale with multiple indices and shards
• test that documentation is self-explanatory and sufficient
• verify the reservior sampling implementation (mix of slow and fast traces)
• test on ECE/ESS
• ensure tail sampling works for apm integration (config options need to be supported)

[simitt]

Closing this, as the preliminary testing during 7.16 cycle was done and a follow up issue is created in #6894.