Report tasks metrics to Prometheus (#5619)

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

substrate/client/service/src/task_manager/prometheus_future.rs

@@ -0,0 +1,69 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+//! Wrapper around a `Future` that reports statistics about when the `Future` is polled.
+
+use futures::prelude::*;
+use prometheus_endpoint::{Counter, Histogram, U64};
+use std::{fmt, pin::Pin, task::{Context, Poll}};
+
+/// Wraps around a `Future`. Report the polling duration to the `Histogram` and when the polling
+/// starts to the `Counter`.
+pub fn with_poll_durations<T>(
+	poll_duration: Histogram,
+	poll_start: Counter<U64>,
+	inner: T
+) -> PrometheusFuture<T> {
+	PrometheusFuture {
+		inner,
+		poll_duration,
+		poll_start,
+	}
+}
+
+/// Wraps around `Future` and adds diagnostics to it.
+#[pin_project::pin_project]
+#[derive(Clone)]
+pub struct PrometheusFuture<T> {
+	/// The inner future doing the actual work.
+	#[pin]
+	inner: T,
+	poll_duration: Histogram,
+	poll_start: Counter<U64>,
+}
+
+impl<T> Future for PrometheusFuture<T>
+where
+	T: Future,
+{
+	type Output = T::Output;
+
+	fn poll(self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
+		let this = self.project();
+
+		this.poll_start.inc();
+		let _timer = this.poll_duration.start_timer();
+		Future::poll(this.inner, cx)
+
+		// `_timer` is dropped here and will observe the duration
+	}
+}
+
+impl<T> fmt::Debug for PrometheusFuture<T>
+where
+	T: fmt::Debug,
+{
+	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+		fmt::Debug::fmt(&self.inner, f)
+	}
+}