Allow W3C TraceContext propagation in kamon-kafka

[Symbianx]

Fixes #1235 by adding a new configuration key kamon.instrumentation.kafka.client.tracing.propagator that can take the value w3c to propagate the trace context using the W3C TraceContext specification.

Please excuse any bad scala code, I'm still a newb in this ecosystem.

[Symbianx]

Haven't done tests yet because I'm having issues compiling kamon-core which fails with unresolved dependencies for javax.annotation.Generated and sun.misc.Contended.

I have I feeling I'm using the wrong sbt or java versions but can't find any information on what settings I should be using to compile the project.

Turns out I need to build using java 8, or at least that did it for me. Will work on adding tests.

[Symbianx]

Added a tests to assert w3c is being used when configured and rebased the PR into 1 commit.

[ivantopo]

Hey @Symbianx, thanks so much for the PR! There is a tiny change required for this to compile on early Scala versions and other than that, it looks good to me.

It would be nice to propagate context tags as W3C baggage so that they don't get lost at this point, but I think that's something that should be addressed on a separate PR.

[ivantopo]

Thanks @Symbianx!

Turns out I already opened an issue regarding w3c-baggage: #803. Let's follow up on that over there.

[Symbianx]

Thank you for merging @ivantopo! 🎉
When can we expect this change to be released? Is there a release cadence we should be aware of?