Add metrics for current number of HTTP client connections.

docs/prometheus-metrics.md

@@ -48,3 +48,4 @@ The following metrics are available:
 | `signaling_grpc_clients`                          | Gauge     | 1.0.0     | The current number of GRPC clients                                        |                                   |
 | `signaling_grpc_client_calls_total`               | Counter   | 1.0.0     | The total number of GRPC client calls                                     | `method`                          |
 | `signaling_grpc_server_calls_total`               | Counter   | 1.0.0     | The total number of GRPC server calls                                     | `method`                          |
+| `signaling_http_client_pool_connections`          | Gauge     | 1.2.4     | The current number of HTTP client connections per host                    | `host`                            |

http_client_pool.go

@@ -29,10 +29,18 @@ import (
 	"net/http"
 	"net/url"
 	"sync"
+
+	"github.com/prometheus/client_golang/prometheus"
 )
 
+func init() {
+	RegisterHttpClientPoolStats()
+}
+
 type Pool struct {
 	pool chan *http.Client
+
+	currentConnections prometheus.Gauge
 }
 
 func (p *Pool) get(ctx context.Context) (client *http.Client, err error) {
@@ -40,21 +48,24 @@ func (p *Pool) get(ctx context.Context) (client *http.Client, err error) {
 	case <-ctx.Done():
 		return nil, ctx.Err()
 	case client := <-p.pool:
+		p.currentConnections.Inc()
 		return client, nil
 	}
 }
 
 func (p *Pool) Put(c *http.Client) {
+	p.currentConnections.Dec()
 	p.pool <- c
 }
 
-func newPool(constructor func() *http.Client, size int) (*Pool, error) {
+func newPool(host string, constructor func() *http.Client, size int) (*Pool, error) {
 	if size <= 0 {
 		return nil, fmt.Errorf("can't create empty pool")
 	}
 
 	p := &Pool{
-		pool: make(chan *http.Client, size),
+		pool:               make(chan *http.Client, size),
+		currentConnections: connectionsPerHostCurrent.WithLabelValues(host),
 	}
 	for size > 0 {
 		c := constructor()
@@ -103,7 +114,7 @@ func (p *HttpClientPool) getPool(url *url.URL) (*Pool, error) {
 		return pool, nil
 	}
 
-	pool, err := newPool(func() *http.Client {
+	pool, err := newPool(url.Host, func() *http.Client {
 		return &http.Client{
 			Transport: p.transport,
 			// Only send body in redirect if going to same scheme / host.

http_client_pool_stats_prometheus.go

@@ -0,0 +1,43 @@
+/**
+ * Standalone signaling server for the Nextcloud Spreed app.
+ * Copyright (C) 2024 struktur AG
+ *
+ * @author Joachim Bauch <bauch@struktur.de>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package signaling
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+var (
+	connectionsPerHostCurrent = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "signaling",
+		Subsystem: "http_client_pool",
+		Name:      "connections",
+		Help:      "The current number of HTTP client connections per host",
+	}, []string{"host"})
+
+	httpClientPoolStats = []prometheus.Collector{
+		connectionsPerHostCurrent,
+	}
+)
+
+func RegisterHttpClientPoolStats() {
+	registerAll(httpClientPoolStats...)
+}