Setup basic prom metrics endpoint (#624)

* Setup basic prom metrics endpoint
* Use default prom handler to expose go runtime metrics

pkg/metrics/metrics.go

@@ -0,0 +1,39 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	sessionsGauge = promauto.NewGauge(prometheus.GaugeOpts{
+		Name: "pgweb_sessions_count",
+		Help: "Total number of database sessions",
+	})
+
+	queriesCounter = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "pgweb_queries_count",
+		Help: "Total number of custom queries executed",
+	})
+
+	healtyGauge = promauto.NewGauge(prometheus.GaugeOpts{
+		Name: "pgweb_healty",
+		Help: "Server health status",
+	})
+)
+
+func IncrementQueriesCount() {
+	queriesCounter.Inc()
+}
+
+func SetSessionsCount(val int) {
+	sessionsGauge.Set(float64(val))
+}
+
+func SetHealty(val bool) {
+	healthy := 0.0
+	if val {
+		healthy = 1.0
+	}
+	healtyGauge.Set(float64(healthy))
+}

pkg/metrics/server.go

@@ -0,0 +1,19 @@
+package metrics
+
+import (
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+	"github.com/sirupsen/logrus"
+)
+
+func Handler() http.Handler {
+	return promhttp.Handler()
+}
+
+func StartServer(logger *logrus.Logger, path string, addr string) error {
+	logger.WithField("addr", addr).WithField("path", path).Info("starting prometheus metrics server")
+
+	http.Handle(path, Handler())
+	return http.ListenAndServe(addr, nil)
+}