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>

substrate/client/rpc-servers/Cargo.toml

@@ -26,5 +26,4 @@ tower = { version = "0.4.13", features = ["util"] }
 http = "0.2.8"
 hyper = "0.14.27"
 futures = "0.3.29"
-pin-project = "1.1.3"
 governor = "0.6.0"