rpc server: add prometheus label is_rate_limited (#3504)

After some discussion with @kogeler after the we added the rate-limit
middleware it may slow down
the rpc call timings metrics significantly because it works as follows:

1. The rate limit guard is checked when the call comes and if a slot is
available -> process the call
2. If no free spot is available then the call will be sleeping
`jitter_delay + min_time_rate_guard` then woken up and checked at most
ten times
3. If no spot is available after 10 iterations -> the call is rejected
(this may take tens of seconds)

Thus, this PR adds a label "is_rate_limited" to filter those out on the
metrics "substrate_rpc_calls_time" and "substrate_rpc_calls_finished".

I had to merge two middleware layers Metrics and RateLimit to avoid
shared state in a hacky way.

---------

Co-authored-by: James Wilson <james@jsdw.me>

Cargo.lock

@@ -16745,7 +16745,6 @@ dependencies = [
  "hyper",
  "jsonrpsee",
  "log",
- "pin-project",
  "serde_json",
  "substrate-prometheus-endpoint",
  "tokio",