propagate context when handling unary Akka gRPC server requests

[ivantopo]

This PR ensures that context (including the HTTP Server Span) that we create automatically on the Akka HTTP instrumentation will get propagated to the service implementation calls.

Notes:

• It only works with unary requests. Stream-based requests will still require manual handling by users.
• The culprit of this issue is that CompletionStage is not instrumented at all and the Java-based version of Akka gRPC uses completable futures in a few places. Writing instrumentation for CompletionStage might be enough to remove this instrumentation in the future.