Provide metric to see processing time by operation name or operation hash

[cernst11]

Currently the apollo_router_processing_time reports all operations to the same histogram bucket and currently the router configuration does not allow for adding custom attributes to the Apollo metrics so currently this metric is impossible to track without going into individual traces.

I would like to see another optional metric to report the apollo_router_processing_time by the operationName or hash in order to easily visualize which operation are causing the router to spend the most processing time.