gRFC L7: Go Metadata API Change

[Doug Fawley]

Discussion for https://github.com/grpc/proposal/pull/25 to change the Go Metadata API in a breaking way for security reasons.

We propose to remove the FromContext and NewContext functions from the metadata package.  These functions imply a context only has one set of metadata, but gRPC must distinguish between incoming (server received) and outgoing (client sending) metadata to avoid accidental forwarding.

This is a breaking change, and will require all users using metadata to inspect their code and update it appropriately.