feat(session): track active sessions

This changes adds a prometheus guage that can be used to approximate
the number of sessions for a given page or country.

README.md

@@ -74,3 +74,13 @@ pages_page_session_seconds_bucket{country="",path="/",le="+Inf"} 1
 pages_page_session_seconds_sum{country="",path="/"} 1.855976549
 pages_page_session_seconds_count{country="",path="/"} 1
 ```
+
+### pages_page_sessions_active
+
+A gauge that approximates the current number of sessions per page.
+
+```text
+# HELP pages_page_sessions_active the number of current sessions for a given page
+# TYPE pages_page_sessions_active gauge
+pages_page_sessions_active{country="",path="/"} 1
+```

internal/metrics/metrics.go

@@ -45,4 +45,14 @@ var (
 		},
 		[]string{"path", "country"},
 	)
+
+	PageSessionsActive = promauto.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: page,
+			Name:      "sessions_active",
+			Help:      "the number of current sessions for a given page",
+		},
+		[]string{"path", "country"},
+	)
 )

internal/session/handler.go

@@ -46,16 +46,20 @@ type Handle struct {
 
 func (h *Handle) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	conn, err := h.upgrader.Upgrade(w, r, nil)
-	defer func() { _ = conn.Close() }()
-
 	if err != nil {
 		// log
 		return
 	}
 
+	defer func() { _ = conn.Close() }()
+
 	path := r.URL.Path
 	geoInfo := geoip.Extract(r.Context())
 
+	active := metrics.PageSessionsActive.WithLabelValues(path, geoInfo.CountryCode)
+	active.Inc()
+	defer func() { active.Dec() }()
+
 	start := time.Now()
 	defer func() {
 		metrics.PageSessionDuration.WithLabelValues(path, geoInfo.CountryCode).Observe(time.Since(start).Seconds())