monitoring: rename rpc metrics
monitoring: rename rpc metrics

Improve consistency with other metric names.

AlCutter/trillian,daviddrysdale/trillian,daviddrysdale/trillian,google/trillian,AlCutter/trillian,google/trillian,codingllama/trillian,Martin2112/trillian,Martin2112/trillian,gdbelvin/trillian,RJPercival/trillian,codingllama/trillian,Martin2112/trillian,gdbelvin/trillian,gdbelvin/trillian,daviddrysdale/trillian,RJPercival/trillian