Support grpc custom metadata pass through

[vordimous]

Describe the desired outcome from the user's perspective
As a gRPC service developer, I want the custom metadata added by both the client and server to be made available as it was defined for each other.

Acceptance criteria

• Metadata included in a request from a gRPC client should be visible to the gRPC service that receives the request
• Metadata included in a response from a gRPC service should be visible to the gRPC client that receives the response

Note: this applies to both grpc-grpc proxying and grpc-grpc-kafka-kafka-grpc-grpc proxying.

[hedhyw]

Hello, any updates on this story? Thank you!

[jfallows]

@hedhyw do you still need generic metadata headers to propagate or just custom status description per #988 for now?

[hedhyw]

@hedhyw do you still need generic metadata headers to propagate or just custom status description per #988 for now?

generic metadata is required too, thank you!