request_ref guard

[Hentioe]

I use the following code to match Finch's asynchronous response:

@impl true
def handle_info({{pool, pid}, resp}, state)
      when pool in [Finch.HTTP1.Pool, Finch.HTTP2.Pool] and is_pid(pid) do
# todo...
end

However, since request_ref is opaque, there is no guarantee that this guard will always be reliable.

Could Finch provide a is_request_ref guard? After all, the structure of request_ref is usually only known to the maintainer.

[sneako]

That sounds like a good idea to me. Please submit a PR!