Add support for sourced metrics. (#6895)

* Add support for sourced metrics.

A sourced metric is a metric that obtains its values
from an existing source, rather than the values being
independently recorded. It thus allows collecting
metrics from existing counters or gauges without
having to duplicate them in a dedicated prometheus
counter or gauge (and hence another atomic value).

The first use-case is to feed the bandwidth counters
from libp2p directly into prometheus.

* Tabs, not spaces.

* Tweak bandwidth counter registration.

* Add debug assertion for variable labels and values.

* Document monotonicity requirement for sourced counters.

* CI

* Update client/network/src/service.rs

Co-authored-by: Max Inden <mail@max-inden.de>

Co-authored-by: Max Inden <mail@max-inden.de>

substrate/utils/prometheus/src/lib.rs

@@ -31,6 +31,9 @@ use std::net::SocketAddr;
 
 #[cfg(not(target_os = "unknown"))]
 mod networking;
+mod sourced;
+
+pub use sourced::{SourcedCounter, SourcedGauge, MetricSource};
 
 #[cfg(target_os = "unknown")]
 pub use unknown_os::init_prometheus;

substrate/utils/prometheus/src/sourced.rs

@@ -0,0 +1,143 @@
+// 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.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Metrics that are collected from existing sources.
+
+use prometheus::core::{Collector, Desc, Describer, Number, Opts};
+use prometheus::proto;
+use std::{cmp::Ordering, marker::PhantomData};
+
+/// A counter whose values are obtained from an existing source.
+///
+/// > **Note*: The counter values provided by the source `S`
+/// > must be monotonically increasing. Otherwise use a
+/// > [`SourcedGauge`] instead.
+pub type SourcedCounter<S> = SourcedMetric<Counter, S>;
+
+/// A gauge whose values are obtained from an existing source.
+pub type SourcedGauge<S> = SourcedMetric<Gauge, S>;
+
+/// The type of a sourced counter.
+#[derive(Copy, Clone)]
+pub enum Counter {}
+
+/// The type of a sourced gauge.
+#[derive(Copy, Clone)]
+pub enum Gauge {}
+
+/// A metric whose values are obtained from an existing source,
+/// instead of being independently recorded.
+#[derive(Debug, Clone)]
+pub struct SourcedMetric<T, S> {
+	source: S,
+	desc: Desc,
+	_type: PhantomData<T>,
+}
+
+/// A source of values for a [`SourcedMetric`].
+pub trait MetricSource: Sync + Send + Clone {
+	/// The type of the collected values.
+	type N: Number;
+	/// Collects the current values of the metrics from the source.
+	fn collect(&self, set: impl FnMut(&[&str], Self::N));
+}
+
+impl<T: SourcedType, S: MetricSource> SourcedMetric<T, S> {
+	/// Creates a new metric that obtains its values from the given source.
+	pub fn new(opts: &Opts, source: S) -> prometheus::Result<Self> {
+		let desc = opts.describe()?;
+		Ok(Self { source, desc, _type: PhantomData })
+	}
+}
+
+impl<T: SourcedType, S: MetricSource> Collector for SourcedMetric<T, S> {
+	fn desc(&self) -> Vec<&Desc> {
+		vec![&self.desc]
+	}
+
+	fn collect(&self) -> Vec<proto::MetricFamily> {
+		let mut counters = Vec::new();
+
+		self.source.collect(|label_values, value| {
+			let mut m = proto::Metric::default();
+
+			match T::proto() {
+				proto::MetricType::COUNTER => {
+					let mut c = proto::Counter::default();
+					c.set_value(value.into_f64());
+					m.set_counter(c);
+				}
+				proto::MetricType::GAUGE => {
+					let mut g = proto::Gauge::default();
+					g.set_value(value.into_f64());
+					m.set_gauge(g);
+				}
+				t => {
+					log::error!("Unsupported sourced metric type: {:?}", t);
+				}
+			}
+
+			debug_assert_eq!(self.desc.variable_labels.len(), label_values.len());
+			match self.desc.variable_labels.len().cmp(&label_values.len()) {
+				Ordering::Greater =>
+					log::warn!("Missing label values for sourced metric {}", self.desc.fq_name),
+				Ordering::Less =>
+					log::warn!("Too many label values for sourced metric {}", self.desc.fq_name),
+				Ordering::Equal => {}
+			}
+
+			m.set_label(self.desc.variable_labels.iter().zip(label_values)
+				.map(|(l_name, l_value)| {
+					let mut l = proto::LabelPair::default();
+					l.set_name(l_name.to_string());
+					l.set_value(l_value.to_string());
+					l
+				})
+				.chain(self.desc.const_label_pairs.iter().cloned())
+				.collect::<Vec<_>>());
+
+			counters.push(m);
+		});
+
+		let mut m = proto::MetricFamily::default();
+		m.set_name(self.desc.fq_name.clone());
+		m.set_help(self.desc.help.clone());
+		m.set_field_type(T::proto());
+		m.set_metric(counters);
+
+		vec![m]
+	}
+}
+
+/// Types of metrics that can obtain their values from an existing source.
+pub trait SourcedType: private::Sealed + Sync + Send {
+	#[doc(hidden)]
+	fn proto() -> proto::MetricType;
+}
+
+impl SourcedType for Counter {
+	fn proto() -> proto::MetricType { proto::MetricType::COUNTER }
+}
+
+impl SourcedType for Gauge {
+	fn proto() -> proto::MetricType { proto::MetricType::GAUGE }
+}
+
+mod private {
+	pub trait Sealed {}
+	impl Sealed for super::Counter {}
+	impl Sealed for super::Gauge {}
+}