NewHandler wraps h to produce metrics on the default engine for every request received and every response sent.

NewHandlerWith wraps h to produce metrics on eng for every request received and every response sent.

NewTransport wraps t to produce metrics on the default engine for every request sent and every response received.

NewTransportWith wraps t to produce metrics on eng for every request sent and every response received.

RequestTags returns the tags associated with the request, if any.

RequestWithTags returns a shallow copy of req with its context updated to include the given tags.