Have more tags in the in_flight_requests meter (eg "service")

The host tag is not currently included, but could be added later with
some refactorings if we need it.

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, 'b> {
+	metrics: &'a HttpsMetrics,
+	tags: &'b [KeyValue],
+}
+
+impl<'a, 'b> Drop for InFlightGuard<'a, 'b> {
+	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 +194,7 @@ async fn handle_request(
 			.unwrap_or_default(),
 	);
 
-	metrics
-		.requests_received
-		.add(1, &[host_tag, method_tag.clone()]);
+	metrics.requests_received.add(1, &[host_tag, method_tag.clone()]);
 
 	let mut tags = vec![method_tag];
 	let resp = select_target_and_proxy(
@@ -272,6 +286,17 @@ async fn select_target_and_proxy(
 		);
 		proxy_to.calls_in_progress.fetch_add(1, Ordering::SeqCst);
 
+		let tags_in_flight = &tags.clone();
+		metrics.requests_in_flight.add(1, &tags_in_flight);
+		// The guard ensures that we decrement requests_in_flight in all cases where
+		// the current tasks ends, including the case where it gets canceled and
+		// doesn't run to completion (which may happen e.g. if it timeouts).
+		// (Crucially we create the guard before the first .await in this function.)
+		let _guard = InFlightGuard {
+			metrics: &metrics,
+			tags: &tags_in_flight,
+		};
+
 		// Forward to backend
 		debug!("{}{} -> {}", host, path, proxy_to);
 		trace!("Request: {:?}", req);