grpc_service: initial_metadata support. (#500)
grpc_service: initial_metadata support. (#500)

Useful when supporting simple header value auth schemes across gRPC
client types.

Signed-off-by: Harvey Tuch <c185081708e02336640916420d0c772668570de5@google.com>
lyft/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,lyft/envoy-api,lyft/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api