Add header logging to calls

To make debugging easier, there should be some way to log metadata that comes with Client Calls, and possibly Server calls.

Start by tracing how Client Calls and Server calls are represented in grpc-java and where their metadata is available. Clarify whether both call directions are required, which metadata should be logged, and the expected logging behavior before identifying an implementation entry point. Done should provide a defined way to log call metadata for the agreed scope.

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