feat(oohelperd): add prometheus metrics (#897)

Closes https://github.com/ooni/probe/issues/2183

While there, avoid exposing nil values for optional fields of the
THResponse struct (i.e., "ip_info" and "tls_handshake").

While there, fix `measurexlite`'s `OperationLogger` test
and make it deterministic rather than racy.

internal/cmd/oohelperd/handler.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"time"
 
 	"github.com/ooni/probe-cli/v3/internal/atomicx"
 	"github.com/ooni/probe-cli/v3/internal/model"
@@ -45,29 +46,40 @@ var _ http.Handler = &handler{}
 
 // ServeHTTP implements http.Handler.ServeHTTP.
 func (h *handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
+	metricRequestsInflight.Inc()
+	defer metricRequestsInflight.Dec()
+	metricRequestsTotal.Inc()
 	w.Header().Add("Server", fmt.Sprintf(
 		"oohelperd/%s ooniprobe-engine/%s", version.Version, version.Version,
 	))
 	if req.Method != "POST" {
+		metricRequestsByStatusCode.WithLabelValues("400").Inc()
 		w.WriteHeader(400)
 		return
 	}
 	reader := &io.LimitedReader{R: req.Body, N: h.MaxAcceptableBody}
 	data, err := netxlite.ReadAllContext(req.Context(), reader)
 	if err != nil {
+		metricRequestsByStatusCode.WithLabelValues("400").Inc()
 		w.WriteHeader(400)
 		return
 	}
 	var creq ctrlRequest
 	if err := json.Unmarshal(data, &creq); err != nil {
+		metricRequestsByStatusCode.WithLabelValues("400").Inc()
 		w.WriteHeader(400)
 		return
 	}
+	started := time.Now()
 	cresp, err := measure(req.Context(), h, &creq)
+	elapsed := time.Since(started)
+	metricMeasurementTime.Observe(float64(elapsed.Seconds()))
 	if err != nil {
+		metricRequestsByStatusCode.WithLabelValues("400").Inc()
 		w.WriteHeader(400)
 		return
 	}
+	metricRequestsByStatusCode.WithLabelValues("200").Inc()
 	// We assume that the following call cannot fail because it's a
 	// clearly-serializable data structure.
 	data, err = json.Marshal(cresp)

internal/cmd/oohelperd/main.go

@@ -14,12 +14,13 @@ import (
 	"github.com/ooni/probe-cli/v3/internal/model"
 	"github.com/ooni/probe-cli/v3/internal/netxlite"
 	"github.com/ooni/probe-cli/v3/internal/runtimex"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
 const maxAcceptableBody = 1 << 24
 
 var (
-	endpoint  = flag.String("endpoint", "127.0.0.1:8080", "Endpoint where to listen")
+	endpoint  = flag.String("endpoint", "127.0.0.1:8080", "API endpoint")
 	srvAddr   = make(chan string, 1) // with buffer
 	srvCancel context.CancelFunc
 	srvCtx    context.Context
@@ -49,6 +50,7 @@ func main() {
 		true:  log.DebugLevel,
 		false: log.InfoLevel,
 	}
+	prometheus := flag.String("prometheus", "127.0.0.1:9091", "Prometheus endpoint")
 	debug := flag.Bool("debug", false, "Toggle debug mode")
 	flag.Parse()
 	log.SetLevel(logmap[*debug])
@@ -75,8 +77,13 @@ func main() {
 	srvAddr <- listener.Addr().String()
 	srvWg.Add(1)
 	go srv.Serve(listener)
+	promMux := http.NewServeMux()
+	promMux.Handle("/metrics", promhttp.Handler())
+	promSrv := &http.Server{Addr: *prometheus, Handler: promMux}
+	go promSrv.ListenAndServe()
 	<-srvCtx.Done()
 	shutdown(srv)
+	shutdown(promSrv)
 	listener.Close()
 	srvWg.Done()
 }

internal/cmd/oohelperd/metrics.go

@@ -0,0 +1,45 @@
+package main
+
+//
+// Metrics definitions
+//
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	// metricRequestsTotal counts the total number of requests
+	metricRequestsTotal = promauto.NewCounter(prometheus.CounterOpts{
+		Name: "oohelperd_requests_total",
+		Help: "The total number of processed requests",
+	})
+
+	// metricRequestsByStatusCode counts the number of requests that
+	// have returned a given status code to the caller.
+	metricRequestsByStatusCode = promauto.NewCounterVec(prometheus.CounterOpts{
+		Name: "oohelperd_requests_by_status_code",
+		Help: "Total number of processed requests by status code",
+	}, []string{"code"})
+
+	// metricRequestsInflight counts the number of requests currently inflight.
+	metricRequestsInflight = promauto.NewGauge(prometheus.GaugeOpts{
+		Name: "oohelperd_requests_inflight",
+		Help: "The number or requests currently inflight",
+	})
+
+	// metricMeasurementTime summarizes the time to perform a measurement.
+	metricMeasurementTime = promauto.NewSummary(prometheus.SummaryOpts{
+		Name: "oohelperd_measurement_time",
+		Help: "Summarizes the time to perform a test-helper measurement (in seconds)",
+		// See https://grafana.com/blog/2022/03/01/how-summary-metrics-work-in-prometheus/
+		Objectives: map[float64]float64{
+			0.25: 0.010, // 0.240 <= φ <= 0.260
+			0.5:  0.010, // 0.490 <= φ <= 0.510
+			0.75: 0.010, // 0.740 <= φ <= 0.760
+			0.9:  0.010, // 0.899 <= φ <= 0.901
+			0.99: 0.001, // 0.989 <= φ <= 0.991
+		},
+	})
+)