multi: record the number of active mailboxes

hashmail_server.go

@@ -406,6 +406,8 @@ func (h *hashMailServer) InitStream(
 
 	h.streams[streamID] = freshStream
 
+	mailboxCount.Set(float64(len(h.streams)))
+
 	return &hashmailrpc.CipherInitResp{
 		Resp: &hashmailrpc.CipherInitResp_Success{},
 	}, nil
@@ -478,6 +480,8 @@ func (h *hashMailServer) TearDownStream(ctx context.Context, streamID []byte,
 
 	delete(h.streams, sid)
 
+	mailboxCount.Set(float64(len(h.streams)))
+
 	return nil
 }
 

prometheus.go

@@ -4,9 +4,18 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
+var (
+	// mailboxCount tracks the current number of active mailboxes.
+	mailboxCount = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "hashmail",
+		Name:      "mailbox_count",
+	})
+)
+
 // PrometheusConfig is the set of configuration data that specifies if
 // Prometheus metric exporting is activated, and if so the listening address of
 // the Prometheus server.
@@ -29,6 +38,7 @@ func StartPrometheusExporter(cfg *PrometheusConfig) error {
 	}
 
 	// Next, we'll register all our metrics.
+	prometheus.MustRegister(mailboxCount)
 
 	// Finally, we'll launch the HTTP server that Prometheus will use to
 	// scape our metrics.