Make RPC system configurable per-client and per-method

[coryan]

When defining client metrics and traces we need to include the RPC system used by the underlying transport. Most client libraries use HTTP, but some use gRPC, and some use HTTP for most methods and gRPC for a few methods.

I think we will need to do one of two things:

• Return the RPC system from the transport layer (aka T4) and use that in the annotations for the client layer (T3 and T2), or
• Have the client library provider the RPC system as a RequestOptions extension, like we do for the URL template.

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Could this duration be used on gRPC clients? If so, we should pass in the rpc system value (to new_with_provider?)

Originally posted by @westarle in #4881