Prometheus metrics endpoint

This starts tracking the queue length over time, so we can get some sense of
when the perf server begins to approach capacity. More detailed metrics to be
added later.

Author: Mark-Simulacrum

Cargo.lock

@@ -44,6 +44,7 @@ analyzeme = { git = "https://github.com/rust-lang/measureme" }
 tar = "0.4"
 inferno = { version="0.10", default-features = false }
 mime = "0.3"
+prometheus = "0.11"
 
 [dependencies.collector]
 path = "../collector"

site/Cargo.toml

@@ -1459,6 +1459,25 @@ impl Server {
         false
     }
 
+    async fn handle_metrics(&self, _req: Request) -> Response {
+        use prometheus::Encoder;
+        let data: Arc<InputData> = self.data.read().as_ref().unwrap().clone();
+
+        let mut buffer = Vec::new();
+        let r = prometheus::Registry::new();
+
+        let queue_length = prometheus::IntGauge::new("queue_length", "queue length").unwrap();
+        queue_length.set(data.missing_commits().await.len() as i64);
+
+        r.register(Box::new(queue_length)).unwrap();
+
+        let encoder = prometheus::TextEncoder::new();
+        let metric_families = r.gather();
+        encoder.encode(&metric_families, &mut buffer).unwrap();
+
+        Response::new(buffer.into())
+    }
+
     async fn handle_push(&self, _req: Request) -> Response {
         lazy_static::lazy_static! {
             static ref LAST_UPDATE: Mutex<Option<Instant>> = Mutex::new(None);
@@ -1590,6 +1609,9 @@ async fn serve_req(ctx: Arc<Server>, req: Request) -> Result<Response, ServerErr
         _ => {}
     }
 
+    if req.uri().path() == "/perf/metrics" {
+        return Ok(ctx.handle_metrics(req).await);
+    }
     if req.uri().path() == "/perf/onpush" {
         return Ok(ctx.handle_push(req).await);
     }