[Feature Request] Support TEMPORAL_GRPC_META_<whatever> environment variable

[cretz]

Describe the solution you'd like

We have described how we can accept gRPC metadata (i.e. HTTP headers) via environment variables at temporalio/proposals#95. But this doesn't need to wait on that to be implemented here like the other environment variables that are already here.

Quoting the proposal:

• For gRPC metadata (i.e. HTTP headers), the environment variable format is TEMPORAL_GRPC_META_<name>.
  • <name> is canonicalized into HTTP header format (gRPC libraries do this for you).
  • Like all HTTP headers, comma-delimited values are supported for multi-headers.
  • This does require that environment variable lists be scanned for prefixes. This is deemed an acceptable tradeoff.
  • 💭 Why not TEMPORAL_GRPC_META as a single var that accepts some kind of structured format?
    • There isn't really a good structured format. Comma-delimited key=value would require users to escape commas in the
      value (common in HTTP values), there's not a good delimiter for other-delimited key=value, and requiring a JSON
      object as the env var value is a bit hard to use.
  • 💭 Why not TEMPORAL_GRPC_META_<index> as key: value?
    • Scanning has to happen anyways.
    • TEMPORAL_GRPC_META_AUTHORIZATION as Bearer my-token is cleaner than TEMPORAL_GRPC_META_0 as
      Authorization: Bearer my-token.
    • Users shouldn't have to keep up with indexes.
  • 💭 Why not TEMPORAL_GRPC_META as a multiline set of headers?
    • This can be added later if wanted.
    • People want to be able to set individual headers.
    • If we use traditional header format, requires HTTP header parsing which is not as trivial as it may seem (and
      parsers may not be present/accessible in every standard library). Granted we could accept our own multiline format
      or only support a simple subset of the header format.