set up opentelemetry for metrics

Also adds an `allow_prometheus` option (disabled by default) to expose
a `/metrics` endpoint that returns Prometheus data.

Cargo.lock

@@ -872,12 +872,15 @@ dependencies = [
  "lru-cache",
  "nix",
  "num_cpus",
+ "once_cell",
  "opentelemetry",
  "opentelemetry-jaeger-propagator",
  "opentelemetry-otlp",
+ "opentelemetry-prometheus",
  "opentelemetry_sdk",
  "parking_lot",
  "phf",
+ "prometheus",
  "rand",
  "regex",
  "reqwest",
@@ -1662,6 +1665,19 @@ dependencies = [
  "tonic",
 ]
 
+[[package]]
+name = "opentelemetry-prometheus"
+version = "0.16.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5e1a24eafe47b693cb938f8505f240dc26c71db60df9aca376b4f857e9653ec7"
+dependencies = [
+ "once_cell",
+ "opentelemetry",
+ "opentelemetry_sdk",
+ "prometheus",
+ "protobuf",
+]
+
 [[package]]
 name = "opentelemetry-proto"
 version = "0.6.0"
@@ -1925,6 +1941,21 @@ dependencies = [
  "yansi",
 ]
 
+[[package]]
+name = "prometheus"
+version = "0.13.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "3d33c28a30771f7f96db69893f78b857f7450d7e0237e9c8fc6427a81bae7ed1"
+dependencies = [
+ "cfg-if",
+ "fnv",
+ "lazy_static",
+ "memchr",
+ "parking_lot",
+ "protobuf",
+ "thiserror",
+]
+
 [[package]]
 name = "prost"
 version = "0.12.6"
@@ -1948,6 +1979,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "protobuf"
+version = "2.28.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "106dd99e98437432fed6519dedecfade6a06a73bb7b2a1e019fdd2bee5778d94"
+
 [[package]]
 name = "quick-error"
 version = "1.2.3"

Cargo.toml

@@ -106,12 +106,15 @@ image = { version = "0.25.1", default-features = false, features = ["jpeg", "png
 jsonwebtoken = "9.3.0"
 lru-cache = "0.1.2"
 num_cpus = "1.16.0"
+once_cell = "1.19.0"
 opentelemetry = "0.23.0"
 opentelemetry-jaeger-propagator = "0.2.0"
 opentelemetry-otlp = "0.16.0"
+opentelemetry-prometheus = "0.16.0"
 opentelemetry_sdk = { version = "0.23.0", features = ["rt-tokio"] }
 parking_lot = { version = "0.12.3", optional = true }
 phf = { version = "0.11.2", features = ["macros"] }
+prometheus = "0.13.4"
 rand = "0.8.5"
 regex = "1.10.4"
 reqwest = { version = "0.12.4", default-features = false, features = ["http2", "rustls-tls-native-roots", "socks"] }

src/config.rs

@@ -60,6 +60,8 @@ pub(crate) struct Config {
     #[serde(default = "false_fn")]
     pub(crate) allow_jaeger: bool,
     #[serde(default = "false_fn")]
+    pub(crate) allow_prometheus: bool,
+    #[serde(default = "false_fn")]
     pub(crate) tracing_flame: bool,
     #[serde(default)]
     pub(crate) proxy: ProxyConfig,

src/main.rs

@@ -412,6 +412,15 @@ fn routes(config: &Config) -> Router {
                 .put(c2s::send_state_event_for_empty_key_route),
         );
 
+    let router = if config.allow_prometheus {
+        router.route(
+            "/metrics",
+            get(|| async { observability::METRICS.export() }),
+        )
+    } else {
+        router
+    };
+
     let router = router
         .route(
             "/_matrix/client/r0/rooms/:room_id/initialSync",

src/observability.rs

@@ -3,13 +3,17 @@
 
 use std::{fs::File, io::BufWriter};
 
-use opentelemetry::KeyValue;
-use opentelemetry_sdk::Resource;
+use once_cell::sync::Lazy;
+use opentelemetry::{metrics::MeterProvider, KeyValue};
+use opentelemetry_sdk::{metrics::SdkMeterProvider, Resource};
 use tracing_flame::{FlameLayer, FlushGuard};
 use tracing_subscriber::{layer::SubscriberExt, EnvFilter, Layer, Registry};
 
 use crate::{config::Config, error, utils::error::Result};
 
+/// Globally accessible metrics state
+pub(crate) static METRICS: Lazy<Metrics> = Lazy::new(Metrics::new);
+
 /// Cleans up resources relating to observability when [`Drop`]ped
 pub(crate) struct Guard {
     /// Drop guard used to flush [`tracing_flame`] data on exit
@@ -85,3 +89,43 @@ fn standard_resource() -> Resource {
         env!("CARGO_PKG_NAME"),
     )]))
 }
+
+/// Holds state relating to metrics
+pub(crate) struct Metrics {
+    /// Internal state for OpenTelemetry metrics
+    ///
+    /// We never directly read from [`SdkMeterProvider`], but it needs to
+    /// outlive all calls to `self.otel_state.0.gather()`, otherwise
+    /// metrics collection will fail.
+    otel_state: (prometheus::Registry, SdkMeterProvider),
+}
+
+impl Metrics {
+    /// Initializes metric-collecting and exporting facilities
+    fn new() -> Self {
+        // Set up OpenTelemetry state
+        let registry = prometheus::Registry::new();
+        let exporter = opentelemetry_prometheus::exporter()
+            .with_registry(registry.clone())
+            .build()
+            .expect("exporter configuration should be valid");
+        let provider = SdkMeterProvider::builder()
+            .with_reader(exporter)
+            .with_resource(standard_resource())
+            .build();
+        let _meter = provider.meter(env!("CARGO_PKG_NAME"));
+
+        // TODO: Add some metrics
+
+        Metrics {
+            otel_state: (registry, provider),
+        }
+    }
+
+    /// Export metrics to a string suitable for consumption by e.g. Prometheus
+    pub(crate) fn export(&self) -> String {
+        prometheus::TextEncoder::new()
+            .encode_to_string(&self.otel_state.0.gather())
+            .expect("should be able to encode metrics")
+    }
+}