[META 757] Baggage in APM Agents

[elastic-apm-tech]

See meta issue and spec for the description and details:

• Meta issue: Baggage in APM Agents apm#757
• Spec issue: [META 757] Spec: Baggage in APM Agents apm#758

[JonasKunz]

Implementation plan:

• move capability of starting spans from AbstractSpan to ElasticContext (otherwise baggage updates might be lost) Context propagation refactorings #3206
• move header context propagation responibility from AbstractSpan to ElasticContext, context propagation needs to happen even if no span is active (e.g. propagating just baggage) Context propagation refactorings #3206
• Implement baggage in ElasticContext, implement header propagation for baggage W3C Baggage propagation and tracer APIs #3236
• Adjust our in-process context propagation to propagate contexts instead of spans (e.g. for ExecutorServices) Implemented in-process baggage propagation #3248
• Bridge OpenTelemetry baggage to our internal baggage implementation OpenTelemetry Bridge Baggage support #3249

[JonasKunz]

The implementation will have the following limitations:

• When creating spans via ElasticAPM.currentSpan().createSpan() baggage updates which happened after the activation of the current span will not be visible on the child span. This should not be a problem though, as baggage can only be updated via the OpenTelemetry API. In that case, users should be using the OpenTelemetry API for creating spans aswell.
• The same limitation applies to our OpenTracing bridge
• Baggage propagation support has not been implemented for Kafka. This will be done via [META 764] Replace binary context propagation with UTF8 encoded textual headers for Otel compatibility #3017