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.

[reyang]

@PeterF778 looks like this PR has received sufficient approvals. Would you rebase and resolve the markdownlint error?

[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?