OpenTelemetry: Semantic Conventions for Feature Flags

[zhiyuanliang-ms]

Previous discussion: #412

We used to follow the feature flag in span (defined in OTEL spec 1.27) path, so we build the telemetry story on ActivityEvent of the .NET Activity system.
But, recently, I found in OTEL spec 1.30, "feature flag in span" has been removed. There is only "feature flag in log`. We should revisit the decision we've made.

[thompson-tomo]

Just to clarify

There is only "feature flag in log`.

There is 2 supported use cases:

• attributes can be added to a log record to provide context
• Events can be raised to capture the result

Both of them can be associated with spans, what changed is that there is no longer dedicated spans to be created for the evaluation. If a provider is interacting with a Web Service to do the evaluation, then that can be captured as a http span etc.