fix for w3c propagation of the proper parentSpanId (#986)

[pnerg]

This PR fixes the issue described in #986.
Using the same test rig as in the linked issue/ticket I now get

Got request for '/forward' with 'traceparent' header [None] and parsed/generated context traceId[3e6740b3d532cc6a748754b77c1a3fa1], spanId[8bcb0b40af42c363], parentSpanId[]
Got request for '/hello' with 'traceparent' header [Some(00-3e6740b3d532cc6a748754b77c1a3fa1-8bcb0b40af42c363-00)] and parsed/generated context traceId[3e6740b3d532cc6a748754b77c1a3fa1], spanId[15f8a0e8a247597f], parentSpanId[8bcb0b40af42c363]

Note that the parentSpan sent to /hello now has the same value as the spanId generated when entering /forward

Note: For the sake of the testing I applied the patch from PR #987 as well as otherwise the identifiers are not properly propagated.

[SimunKaracic]

Nice catch and fix! But there's still a test that needs fixing

[pnerg]

Dang, missed that the tests are in kamon-core-tests I only executed sbt kamon-core/test should perhaps have been a bit suspicious as the command executed a bit too fast. 😖

[SimunKaracic]

Np, that's what we have this whole setup for!
Merging as soon as the robots confirm that tests pass

[pnerg]

Failing with [main] WARNING 2021-04-09 11:16:52 Logger : Error trying to load Instrumentation: kamon.instrumentation.akka.instrumentations.akka_26.ActorMonitorInstrumentation with error: java.lang.ClassNotFoundException: kamon.instrumentation.akka.instrumentations.akka_26.ActorMonitorInstrumentation which doesn't seem related to my changes