Prometheus metrics for RPC calls (#7088)

* WS and HTTP middlewares added

* Prometheus endpoint added

* Counters renamed

* Proper style for inc

* Metrics initialization re-written

* Rework to handler middleware

* Introduce transport prefix for metrics

* String shortened

* Commented code removed, new line inserted

* One more string shortened

* Wasm build fixed

* Wasm build fixed once again

* Rework to shared metrics

* Added collectors label

* Tilde removed from cargo

* Switch to owned metrics in parameters

substrate/Cargo.lock

@@ -7111,6 +7111,7 @@ dependencies = [
 name = "sc-rpc-server"
 version = "2.0.0-rc6"
 dependencies = [
+ "futures 0.1.29",
  "jsonrpc-core",
  "jsonrpc-http-server",
  "jsonrpc-ipc-server",
@@ -7120,6 +7121,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sp-runtime",
+ "substrate-prometheus-endpoint",
 ]
 
 [[package]]