Messaging tracing and metrics: adopt OTel semconv 1.28.0+

[lmolkova]

we're going to GA otel tracing plugin, but will keep messaging experimental:

• messaging SDKs should check if experimental mode is enabled and not emit anything if not
• .NET uses AZURE_EXPERIMENTAL_ENABLE_ACTIVITY_SOURCE env var for it, we'll use something like AZURE_ENABLE_EXPERIMENTAL_OBSERVABILITY_FEATURES (tbd and unify across languages)
• we'll need to adopt messaging changes coming with OTel semconv tbd
• apply it to metrics and traces