Add wrap-request-method-counter, update key-fn

Adds middleware for counting HTTP request methods. This middleware has similar
:key-fn option to wrap-response-code-counter.

Changes :key-fn options to be passed the request/response map in
wrap-request-method-counter/wrap-response-code-counter respectively (instead of
status code directly). This allows for more powerful key-fns.

Increments major version because the key-fn update is non-backwards-compatible.