[APM] Support for dropping unsampled transactions (meta)

[graphaelli]

Meta Issue for tracking overall progress on updates for supporting apm-server.sampling.keep_unsampled: false.

• [APM] ML jobs should support metric documents #101734
• [APM] Alerts should support transaction metrics #78122
• [APM] Metric-powered correlations #118437

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

We are not actively working on making correlations work for metrics, so I suggest we close this and declare metric-power ui released.

[axw]

IIANM, the transaction latency distribution chart is not using metrics. Is that as intended? Below is what the transaction page looks like after I sent some transactions, waited for metrics to be aggregated, and then deleted traces-apm-default. I expected the latency distribution to be visible, only with no sample traces.

[sorenlouv]

@axw Thanks for pointing this out. Definitely not intentional. I think we should be able to display the histogram even if correlations doesn't support transaction metrics.

I created a bug report #132394

[dannycroft]

@sqren @dgieselaar what would like to do with this issue. It's marked as ready but #118437 seems to be marked as stale? Shall we bring it to refinement?

[dgieselaar]

@dannycroft I think we can close it and keep the other one open. Perhaps we can discuss the ML one in the APM x ML sync?

[sorenlouv]

Closing this with #132394 as the only immediate follow-up.