Use remote_execution.RequestMetadata

[sluongng]

Currently requests to remote execution platforms do not come with a RequestMetadata

buck2/remote_execution/oss/re_grpc_proto/proto/build/bazel/remote/execution/v2/remote_execution.proto

Line 1883 in aebff47

message RequestMetadata {

. Having this would help RBE services to differentiate a Buck2 remote build from others.

Right now, there is only internal metadata attached to the request

buck2/remote_execution/oss/re_grpc/src/client.rs

Lines 1252 to 1257 in aebff47

	struct Metadata {
	#[prost(message, optional, tag = "15")]
	platform: Option<crate::grpc::Platform>,
	#[prost(string, optional, tag = "18")]
	use_case_id: Option<String>,
	}

.

[ndmitchell]

See #310 where this is being discussed.