Add revision to version string (#3147)

Since the removal of `build.sh`, we don't include any information about
the revision Dendrite was build from. Since go1.18, the revision a
binary was build from is automatically included, so we can try to get
that instead.

This also adds a `dendrite_up` metric showing the current version
(`dendrite_up{version="0.13.1+c796f20"} 1`)

Closes #2993

cmd/dendrite/main.go

@@ -26,6 +26,7 @@ import (
 	"github.com/matrix-org/dendrite/setup/jetstream"
 	"github.com/matrix-org/dendrite/setup/process"
 	"github.com/matrix-org/gomatrixserverlib/fclient"
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/sirupsen/logrus"
 
 	"github.com/matrix-org/dendrite/appservice"
@@ -187,6 +188,16 @@ func main() {
 		}
 	}
 
+	upCounter := prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: "dendrite",
+		Name:      "up",
+		ConstLabels: map[string]string{
+			"version": internal.VersionString(),
+		},
+	})
+	upCounter.Add(1)
+	prometheus.MustRegister(upCounter)
+
 	// Expose the matrix APIs directly rather than putting them under a /api path.
 	go func() {
 		basepkg.SetupAndServeHTTP(processCtx, cfg, routers, httpAddr, nil, nil)