Document gRPC Metadata model and APIs

[hsaliak]

Common Questions:

• What's a use case for metadata? Some examples where this is used, eg auth.

• How do I add metadata? What API to call?

• What are the payloads of metadata? String only, or can it be PB definitions?

• How is it different from Context Propagation?

• [ ] Java

• [ ] C/Core based Wrapped language

• [ ] Go

• [ ] All languages