From 08bbb1537e11be92a7477450b94c75bc18708062 Mon Sep 17 00:00:00 2001
From: Dan Sosedoff <dan.sosedoff@gmail.com>
Date: Thu, 30 Mar 2023 12:51:49 -0500
Subject: [PATCH] Implement process uptime metric (#666)

* Implement process uptime metric
* Add clarification comment
* Add another clarification comment
---
 pkg/api/routes.go      |  3 ++-
 pkg/cli/cli.go         |  2 ++
 pkg/metrics/handler.go | 25 +++++++++++++++++++++++++
 pkg/metrics/metrics.go |  5 +++++
 pkg/metrics/server.go  |  7 +------
 5 files changed, 35 insertions(+), 7 deletions(-)
 create mode 100644 pkg/metrics/handler.go

diff --git a/pkg/api/routes.go b/pkg/api/routes.go
index 5b22695..01aaae9 100644
--- a/pkg/api/routes.go
+++ b/pkg/api/routes.go
@@ -61,6 +61,7 @@ func SetupRoutes(router *gin.Engine) {
 
 func SetupMetrics(engine *gin.Engine) {
 	if command.Opts.MetricsEnabled && command.Opts.MetricsAddr == "" {
-		engine.GET("/metrics", gin.WrapH(metrics.Handler()))
+		// NOTE: We're not supporting the MetricsPath CLI option here to avoid the route conflicts.
+		engine.GET("/metrics", gin.WrapH(metrics.NewHandler()))
 	}
 }
diff --git a/pkg/cli/cli.go b/pkg/cli/cli.go
index 70493a2..0073f79 100644
--- a/pkg/cli/cli.go
+++ b/pkg/cli/cli.go
@@ -307,6 +307,8 @@ func Run() {
 		}
 	}
 
+	// Start a separate metrics http server. If metrics addr is not provided, we
+	// add the metrics endpoint in the existing application server (see api.go).
 	if options.MetricsEnabled && options.MetricsAddr != "" {
 		go startMetricsServer()
 	}
diff --git a/pkg/metrics/handler.go b/pkg/metrics/handler.go
new file mode 100644
index 0000000..0c2b61c
--- /dev/null
+++ b/pkg/metrics/handler.go
@@ -0,0 +1,25 @@
+package metrics
+
+import (
+	"net/http"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus/promhttp"
+)
+
+type Handler struct {
+	startTime   time.Time
+	promHandler http.Handler
+}
+
+func (h Handler) ServeHTTP(rw http.ResponseWriter, req *http.Request) {
+	uptimeGauge.Set(time.Since(h.startTime).Seconds())
+	h.promHandler.ServeHTTP(rw, req)
+}
+
+func NewHandler() http.Handler {
+	return Handler{
+		startTime:   time.Now(),
+		promHandler: promhttp.Handler(),
+	}
+}
diff --git a/pkg/metrics/metrics.go b/pkg/metrics/metrics.go
index 7b8b112..7e0c6d2 100644
--- a/pkg/metrics/metrics.go
+++ b/pkg/metrics/metrics.go
@@ -20,6 +20,11 @@ var (
 		Name: "pgweb_healthy",
 		Help: "Server health status",
 	})
+
+	uptimeGauge = promauto.NewGauge(prometheus.GaugeOpts{
+		Name: "pgweb_uptime",
+		Help: "Server application uptime in seconds",
+	})
 )
 
 func IncrementQueriesCount() {
diff --git a/pkg/metrics/server.go b/pkg/metrics/server.go
index 7291d61..4041e7d 100644
--- a/pkg/metrics/server.go
+++ b/pkg/metrics/server.go
@@ -3,17 +3,12 @@ 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())
+	http.Handle(path, NewHandler())
 	return http.ListenAndServe(addr, nil)
 }