Composite Head Samplers

[PeterF778]

Changes

Migrating from open-telemetry/oteps#250.

This is another approach for introducing Composite (Head) Samplers. The previous one (open-telemetry/oteps#240) proved too large, with some controversial elements.
This OTEP is a split-off from that one, focusing just on one area - new Composite Samplers.

Two prototypes exist for this functionality. We are seeking a third prototype.

In the Java-contrib repo: https://github.com/open-telemetry/opentelemetry-java-contrib/blob/main/consistent-sampling/README.md

In @jmacd's https://github.com/jmacd/go-sampler/blob/main/README.md.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[PeterF778]

Added IsAdjustedCountReliable to SamplingIntent, as discussed by the Sampling SIG.

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[github-actions]

Closed as inactive. Feel free to reopen if this PR is still being worked on.

[carlosalberto]

Overall LGTM, just left a few questions. Also, is the plan that languages implement all of these samplers? There's no requirement anywhere but I'm curious on the expectation, e.g. what if at least consistent and consistentparentbased should be supported?

[PeterF778]

Also, is the plan that languages implement all of these samplers? There's no requirement anywhere but I'm curious on the expectation, e.g. what if at least consistent and consistentparentbased should be supported?

Good question, this has not been discussed yet, but if we assume that the example is realistic, we probably need all of them. I think we should say that an OTel SDK SHOULD implement them all.

[carlosalberto]

Forgot to mention all the sampling OTEPs are under oteps/tracing, so maybe consider moving it there?

[carlosalberto]

I think we can go ahead and merge this @PeterF778 . Probably only consider moving this OTEP under trace/?

[jmacd]

FYI A new prototype has been written in OTel-Rust:
https://github.com/jmacd/rust-sampler

The portion of this OTEP that my two prototypes (Go and Rust) do not include, thus far, is a rate-limited sampler. I will suggest we (sampling SIG) prototype the rate-limited sampler as we work to propose updates to the spec to reflect this OTEP.

Thank you @PeterF778!