Support (optional) Context Propagation

[pjfanning]

Kamon supports Context Propagation. Kamon uses AOP style code instrumentation. See #1487 for a related discussion about metrics gathering.

The question is if we could add something in Pekko itself so that we don't need to have Kamon manipulate Pekko code at runtime.

Maybe, we could allow users to opt in but when they opt in, we could include a Context object like a Map[String, String] with each message sent between actor instances.

https://github.com/open-telemetry/opentelemetry-java-instrumentation also supports context tracing in Pekko and Pekko HTTP and has some similarities to Kamon in the way that it uses byte-buddy to instrument our code at runtime.