Support BINARY format

[carlosalberto]

As per this only TEXT_MAP format is supported.

I think support for BINARY could be added as an option for the user to explicitly specify, and keep TEXT_MAP as the default.

[carlosalberto]

cc @spiliopoulos

[malafeev]

I don't see how we can add support for Binary.
Binary adapters requires ByteBuffer.
We inject/extract context to/from Metadata.
But Metadata doesn't provide access to byte array or byte buffer.

[carlosalberto]

What about https://github.com/grpc/grpc-java/blob/master/api/src/main/java/io/grpc/Metadata.java#L76 ? Would that help? Let me know. If it's truly not possible, lets close this item.

Else we can keep it as a potential enhancement ;) (not an urgent thing, that is)

[malafeev]

I think I need more time to evaluate how binary context can be injected. I didn't see usage of it in java instrumentations.