tracing: Support sampling on Elastic APM

[igorwwwwwwwwwwwwwwwwwwww]

This is a patch to address #2039.

Tracing every request can be quite expensive. By introducing a sample rate (similar to what stackdriver and jaeger already support), the overhead and trace storage cost can be controlled.

This patch adds sampling for Elastic APM tracing.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

• tracing: Support sampling on Elastic APM with new sample_rate setting

Verification

• I still need to run a proper integration test. Is there a standalone-ish way of running thanos locally that would enable such testing?
• Since the default value for sample rate is 0, this will break anyone currently using the elastic APM backend. It might make sense to have a proper default value of 1 -- this applies to various settings on other tracing backends.

[bwplotka]

That was kind of easy (((:

LGTM!

Thanks 👍

[igorwwwwwwwwwwwwwwwwwwww]

Managed to verify that this works.

Surprisingly, the APM library will still emit empty traces for non-sampled events (basically a root span without children), but that appears to be by design.