initial prometheus metrics (#1536)

* service-new: cosmetic changes

* overseer: draft of prometheus metrics

* metrics: update active_leaves metrics

* metrics: extract into functions

* metrics: resolve XXX

* metrics: it's ugly, but it works

* Bump Substrate

* metrics: move a bunch of code around

* Bumb substrate again

* metrics: fix a warning

* fix a warning in runtime

* metrics: statements signed

* metrics: statements impl RegisterMetrics

* metrics: refactor Metrics trait

* metrics: add Metrics assoc type to JobTrait

* metrics: move Metrics trait to util

* metrics: fix overseer

* metrics: fix backing

* metrics: fix candidate validation

* metrics: derive Default

* metrics: docs

* metrics: add stubs for other subsystems

* metrics: add more stubs and fix compilation

* metrics: fix doctest

* metrics: move to subsystem

* metrics: fix candidate validation

* metrics: bitfield signing

* metrics: av store

* metrics: chain API

* metrics: runtime API

* metrics: stub for avad

* metrics: candidates seconded

* metrics: ok I gave up

* metrics: provisioner

* metrics: remove a clone by requiring Metrics: Sync

* metrics: YAGNI

* metrics: remove another TODO

* metrics: for later

* metrics: add parachain_ prefix

* metrics: s/signed_statement/signed_statements

* utils: add a comment for job metrics

* metrics: address review comments

* metrics: oops

* metrics: make sure to save files before commit 😅

* use _total suffix for requests metrics

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

* metrics: add tests for overseer

* update Cargo.lock

* overseer: add a test for CollationGeneration

* collation-generation: impl metrics

* collation-generation: use kebab-case for name

* collation-generation: add a constructor

Co-authored-by: Gav Wood <gavin@parity.io>
Co-authored-by: Ashley Ruglys <ashley.ruglys@gmail.com>
Co-authored-by: Max Inden <mail@max-inden.de>

polkadot/node/subsystem/src/lib.rs

@@ -19,6 +19,8 @@
 //! Node-side logic for Polkadot is mostly comprised of Subsystems, which are discrete components
 //! that communicate via message-passing. They are coordinated by an overseer, provided by a
 //! separate crate.
+//!
+//! This crate also reexports Prometheus metric types which are expected to be implemented by subsystems.
 
 #![warn(missing_docs)]
 
@@ -128,9 +130,9 @@ impl From<oneshot::Canceled> for SubsystemError {
 }
 
 impl From<futures::task::SpawnError> for SubsystemError {
-    fn from(_: futures::task::SpawnError) -> Self {
+	fn from(_: futures::task::SpawnError) -> Self {
 		Self
-    }
+	}
 }
 
 impl From<std::convert::Infallible> for SubsystemError {
@@ -202,6 +204,9 @@ pub trait SubsystemContext: Send + 'static {
 /// [`Overseer`]: struct.Overseer.html
 /// [`Subsystem`]: trait.Subsystem.html
 pub trait Subsystem<C: SubsystemContext> {
+	/// Subsystem-specific Prometheus metrics.
+	type Metrics: metrics::Metrics;
+
 	/// Start this `Subsystem` and return `SpawnedSubsystem`.
 	fn start(self, ctx: C) -> SpawnedSubsystem;
 }
@@ -211,6 +216,8 @@ pub trait Subsystem<C: SubsystemContext> {
 pub struct DummySubsystem;
 
 impl<C: SubsystemContext> Subsystem<C> for DummySubsystem {
+	type Metrics = ();
+
 	fn start(self, mut ctx: C) -> SpawnedSubsystem {
 		let future = Box::pin(async move {
 			loop {
@@ -227,4 +234,41 @@ impl<C: SubsystemContext> Subsystem<C> for DummySubsystem {
 			future,
 		}
 	}
-}
+}
+
+/// This module reexports Prometheus types and defines the [`Metrics`] trait.
+pub mod metrics {
+	/// Reexport Prometheus types.
+	pub use substrate_prometheus_endpoint as prometheus;
+
+	/// Subsystem- or job-specific Prometheus metrics.
+	///
+	/// Usually implemented as a wrapper for `Option<ActualMetrics>`
+	/// to ensure `Default` bounds or as a dummy type ().
+	/// Prometheus metrics internally hold an `Arc` reference, so cloning them is fine.
+	pub trait Metrics: Default + Clone {
+		/// Try to register metrics in the Prometheus registry.
+		fn try_register(registry: &prometheus::Registry) -> Result<Self, prometheus::PrometheusError>;
+
+		/// Convience method to register metrics in the optional Prometheus registry.
+		/// If the registration fails, prints a warning and returns `Default::default()`.
+		fn register(registry: Option<&prometheus::Registry>) -> Self {
+			registry.map(|r| {
+				match Self::try_register(r) {
+					Err(e) => {
+						log::warn!("Failed to register metrics: {:?}", e);
+						Default::default()
+					},
+					Ok(metrics) => metrics,
+				}
+			}).unwrap_or_default()
+		}
+	}
+
+	// dummy impl
+	impl Metrics for () {
+		fn try_register(_registry: &prometheus::Registry) -> Result<(), prometheus::PrometheusError> {
+			Ok(())
+		}
+	}
+}