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/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
+		},
+	})
+)