Stabilize OpenTelemetry TraceState handling ("ot" key and sub-keys "th" and "rv")

[jmacd]

What are you trying to achieve?

OpenTelemetry defines its probability sampling specification expressed in terms of its TraceState value, primarily in this document:

https://opentelemetry.io/docs/specs/otel/trace/tracestate-handling/

What did you expect to see?

The Sampling SIG believes this is ready to move out of development. It has been implemented multiple times including the OpenTelemetry Collector-Contrib's probabilistic sampling processor, at least one vendor backend, and multiple SDK sampler prototypes in support of OTEP 250.

https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/processor/probabilisticsamplerprocessor/README.md

Additional context.

https://github.com/open-telemetry/opentelemetry-collector-contrib/blob/main/pkg/sampling/README.md