Support multiple urgencies for Grape endpoints with the same path but different HTTP verb

Currently, it's only possible to set an urgency for an endpoint by path. As a result, if there are 2 endpoints with the same path, distinguished by the HTTP verb, they need to have the same urgency.

This happened in gitlab-org/gitlab!85339 (comment 917868691) where I suggested using the urgency from the busier endpoint.

If this is a problem for multiple routes, we should add support for different urgencies by path/verb combination, and not just by path. Otherwise, we should document what to do if there is a conflict like this.