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 <bkchr@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* move log to separate lines.

* Fix compilation.

* cargo +nightly fmt --all

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/rpc-servers/src/lib.rs

@@ -24,6 +24,7 @@ mod middleware;
 
 use jsonrpc_core::{IoHandlerExtension, MetaIoHandler};
 use log::error;
+use prometheus_endpoint::{register, Counter, PrometheusError, Registry, U64};
 use pubsub::PubSubMetadata;
 use std::io;
 
@@ -42,7 +43,7 @@ const HTTP_THREADS: usize = 4;
 pub type RpcHandler<T> = pubsub::PubSubHandler<T, RpcMiddleware>;
 
 pub use self::inner::*;
-pub use middleware::{RpcMetrics, RpcMiddleware};
+pub use middleware::{method_names, RpcMetrics, RpcMiddleware};
 
 /// Construct rpc `IoHandler`
 pub fn rpc_handler<M: PubSubMetadata>(
@@ -73,6 +74,43 @@ pub fn rpc_handler<M: PubSubMetadata>(
 	io
 }
 
+/// RPC server-specific prometheus metrics.
+#[derive(Debug, Clone, Default)]
+pub struct ServerMetrics {
+	/// Number of sessions opened.
+	session_opened: Option<Counter<U64>>,
+	/// Number of sessions closed.
+	session_closed: Option<Counter<U64>>,
+}
+
+impl ServerMetrics {
+	/// Create new WebSocket RPC server metrics.
+	pub fn new(registry: Option<&Registry>) -> Result<Self, PrometheusError> {
+		registry
+			.map(|r| {
+				Ok(Self {
+					session_opened: register(
+						Counter::new(
+							"rpc_sessions_opened",
+							"Number of persistent RPC sessions opened",
+						)?,
+						r,
+					)?
+					.into(),
+					session_closed: register(
+						Counter::new(
+							"rpc_sessions_closed",
+							"Number of persistent RPC sessions closed",
+						)?,
+						r,
+					)?
+					.into(),
+				})
+			})
+			.unwrap_or_else(|| Ok(Default::default()))
+	}
+}
+
 #[cfg(not(target_os = "unknown"))]
 mod inner {
 	use super::*;
@@ -84,6 +122,16 @@ mod inner {
 	/// Type alias for ws server
 	pub type WsServer = ws::Server;
 
+	impl ws::SessionStats for ServerMetrics {
+		fn open_session(&self, _id: ws::SessionId) {
+			self.session_opened.as_ref().map(|m| m.inc());
+		}
+
+		fn close_session(&self, _id: ws::SessionId) {
+			self.session_closed.as_ref().map(|m| m.inc());
+		}
+	}
+
 	/// Start HTTP server listening on given address.
 	///
 	/// **Note**: Only available if `not(target_os = "unknown")`.
@@ -114,6 +162,7 @@ mod inner {
 	pub fn start_ipc<M: pubsub::PubSubMetadata + Default>(
 		addr: &str,
 		io: RpcHandler<M>,
+		server_metrics: ServerMetrics,
 	) -> io::Result<ipc::Server> {
 		let builder = ipc::ServerBuilder::new(io);
 		#[cfg(target_os = "unix")]
@@ -122,7 +171,7 @@ mod inner {
 			security_attributes.set_mode(0o600)?;
 			security_attributes
 		});
-		builder.start(addr)
+		builder.session_stats(server_metrics).start(addr)
 	}
 
 	/// Start WS server listening on given address.
@@ -136,6 +185,7 @@ mod inner {
 		cors: Option<&Vec<String>>,
 		io: RpcHandler<M>,
 		maybe_max_payload_mb: Option<usize>,
+		server_metrics: ServerMetrics,
 	) -> io::Result<ws::Server> {
 		let rpc_max_payload = maybe_max_payload_mb
 			.map(|mb| mb.saturating_mul(MEGABYTE))
@@ -147,6 +197,7 @@ mod inner {
 		.max_connections(max_connections.unwrap_or(WS_MAX_CONNECTIONS))
 		.allowed_origins(map_cors(cors))
 		.allowed_hosts(hosts_filtering(cors.is_some()))
+		.session_stats(server_metrics)
 		.start(addr)
 		.map_err(|err| match err {
 			ws::Error::Io(io) => io,