Better RPC prometheus metrics. (#9358)

* Better RPC prometehus metrics.

* Add session metrics.

* Add counting requests as well.

* Fix type for web build.

* Fix browser-node

* Filter out unknown method names.

* Change Gauge to Counters

* Use micros instead of millis.

* cargo fmt

* Update client/rpc-servers/src/lib.rs

Co-authored-by: Bastian Köcher <[email protected]>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <[email protected]>

* move log to separate lines.

* Fix compilation.

* cargo +nightly fmt --all

Co-authored-by: Bastian Köcher <[email protected]>