Add a metrics counter measuring in-flight requests

src/https.rs

@@ -41,9 +41,21 @@ pub struct HttpsConfig {
 struct HttpsMetrics {
 	requests_received: metrics::Counter<u64>,
 	requests_served: metrics::Counter<u64>,
+	requests_in_flight: metrics::UpDownCounter<i64>,
 	request_proxy_duration: metrics::Histogram<f64>,
 }
 
+struct InFlightGuard<'a> {
+	metrics: Arc<HttpsMetrics>,
+	tags: &'a [KeyValue],
+}
+
+impl<'a> Drop for InFlightGuard<'a> {
+	fn drop(&mut self) {
+		self.metrics.requests_in_flight.add(-1, self.tags)
+	}
+}
+
 pub async fn serve_https(
 	config: HttpsConfig,
 	cert_store: Arc<CertStore>,
@@ -62,6 +74,10 @@ pub async fn serve_https(
 			.u64_counter("https_requests_served")
 			.with_description("Total number of requests served over HTTPS")
 			.init(),
+		requests_in_flight: meter
+			.i64_up_down_counter("https_requests_in_flight")
+			.with_description("Current number of requests handled over HTTPS")
+			.init(),
 		request_proxy_duration: meter
 			.f64_histogram("https_request_proxy_duration")
 			.with_description("Duration between time when request was received, and time when backend returned status code and headers")
@@ -178,9 +194,13 @@ async fn handle_request(
 			.unwrap_or_default(),
 	);
 
-	metrics
-		.requests_received
-		.add(1, &[host_tag, method_tag.clone()]);
+	let tags = [host_tag, method_tag.clone()];
+	metrics.requests_received.add(1, &tags);
+	metrics.requests_in_flight.add(1, &tags);
+	let _guard = InFlightGuard {
+		metrics: metrics.clone(),
+		tags: &tags,
+	};
 
 	let mut tags = vec![method_tag];
 	let resp = select_target_and_proxy(