Spec how composite sampler works with legacy head-based sampling

[yuanyuanzhao3]

What are you trying to achieve?
We want to clarify with an example of how head-based sampling works when an upstream legacy sampler makes sampling decisions in a way that is different from OTel consistent probabilistic sampler. As an example, such a legacy sampler may not specify a th value for the ot key in tracestate.

This would give guidance for people on how to configure their samplers in a mixed environment where services ownership is distributed among multiple teams and departments. A coordinated simultaneous upgrade of everything to OTel-compatible is not feasible. During the transitioning time, people will need to deal with the mixed setup and also have a way to obtain valid span metrics.

What did you expect to see?
We would like to see a clear specification, through an example on how the samplers can be configured at an OTel-instrumented service where the upstream sampler may not be OTel-compatible. We expect head-based decisions to be "respected*. There should be clarification on how the sampler output enables valid span metrics computation.

Additional context.

Add any other context about the problem here. If you followed an existing documentation, please share the link to it.