cmd/sh-api: add metric for GraphQL query duration

cmd/sh-api/main.go

@@ -24,6 +24,15 @@ import (
 	"git.sr.ht/~sircmpwn/searchhut/graph/generated"
 )
 
+var durationMetric = prometheus.NewHistogramVec(
+	prometheus.HistogramOpts{
+		Name:    "query_duration_seconds",
+		Help:    "GraphQL query latency in seconds.",
+		Buckets: prometheus.DefBuckets,
+	},
+	nil,
+)
+
 func main() {
 	debug := false
 	opts, _, err := getopt.Getopts(os.Args, "d")
@@ -54,6 +63,8 @@ func main() {
 	}
 	prometheus.DefaultRegisterer.Register(promcollectors.NewDBStatsCollector(db, "main"))
 
+	prometheus.MustRegister(durationMetric)
+
 	schema := generated.NewExecutableSchema(generated.Config{
 		Resolvers: &graph.Resolver{},
 	})
@@ -65,7 +76,7 @@ func main() {
 	router.Use(middleware.Logger)
 	router.Use(middleware.Timeout(10 * time.Second))
 
-	router.Handle("/query", srv)
+	router.With(promQueryMiddleware).Handle("/query", srv)
 	if debug {
 		router.Handle("/", playground.Handler("GraphQL playground", "/query"))
 	}
@@ -79,3 +90,7 @@ func main() {
 	log.Printf("Running server on %s", port)
 	log.Fatal(http.ListenAndServe(port, router))
 }
+
+func promQueryMiddleware(next http.Handler) http.Handler {
+	return promhttp.InstrumentHandlerDuration(durationMetric, next)
+}