Added GetOrCreateGauge to be consistent with Counter and Summary

gauge.go

@@ -19,6 +19,9 @@ import (
 //
 // The returned gauge is safe to use from concurrent goroutines.
 func NewGauge(name string, f func() float64) *Gauge {
+	if f == nil {
+		panic(fmt.Errorf("BUG: f cannot be nil"))
+	}
 	g := &Gauge{
 		f: f,
 	}
@@ -47,3 +50,51 @@ func (g *Gauge) marshalTo(prefix string, w io.Writer) {
 		fmt.Fprintf(w, "%s %g\n", prefix, v)
 	}
 }
+
+// GetOrCreateGauge returns registered gauge with the given name
+// or creates new gauge if the registry doesn't contain gauge with
+// the given name.
+//
+// name must be valid Prometheus-compatible metric with possible lables.
+// For instance,
+//
+//     * foo
+//     * foo{bar="baz"}
+//     * foo{bar="baz",aaa="b"}
+//
+// The returned gauge is safe to use from concurrent goroutines.
+//
+// Performance tip: prefer NewGauge instead of GetOrCreateGauge.
+func GetOrCreateGauge(name string, f func() float64) *Gauge {
+	metricsMapLock.Lock()
+	nm := metricsMap[name]
+	metricsMapLock.Unlock()
+	if nm == nil {
+		// Slow path - create and register missing gauge.
+		if f == nil {
+			panic(fmt.Errorf("BUG: f cannot be nil"))
+		}
+		if err := validateMetric(name); err != nil {
+			panic(fmt.Errorf("BUG: invalid metric name %q: %s", name, err))
+		}
+		nmNew := &namedMetric{
+			name: name,
+			metric: &Gauge{
+				f: f,
+			},
+		}
+		metricsMapLock.Lock()
+		nm = metricsMap[name]
+		if nm == nil {
+			nm = nmNew
+			metricsMap[name] = nm
+			metricsList = append(metricsList, nm)
+		}
+		metricsMapLock.Unlock()
+	}
+	g, ok := nm.metric.(*Gauge)
+	if !ok {
+		panic(fmt.Errorf("BUG: metric %q isn't a Gauge. It is %T", name, nm.metric))
+	}
+	return g
+}

gauge_example_test.go

@@ -16,3 +16,26 @@ func ExampleGauge() {
 	// Obtain gauge value.
 	fmt.Println(g.Get())
 }
+
+func ExampleGauge_vec() {
+	for i := 0; i < 3; i++ {
+		// Dynamically construct metric name and pass it to GetOrCreateGauge.
+		name := fmt.Sprintf(`metric{label1=%q, label2="%d"}`, "value1", i)
+		iLocal := i
+		metrics.GetOrCreateGauge(name, func() float64 {
+			return float64(iLocal + 1)
+		})
+	}
+
+	// Read counter values.
+	for i := 0; i < 3; i++ {
+		name := fmt.Sprintf(`metric{label1=%q, label2="%d"}`, "value1", i)
+		n := metrics.GetOrCreateGauge(name, func() float64 { return 0 }).Get()
+		fmt.Println(n)
+	}
+
+	// Output:
+	// 1
+	// 2
+	// 3
+}

gauge_test.go

@@ -6,6 +6,15 @@ import (
 	"testing"
 )
 
+func TestGaugeError(t *testing.T) {
+	expectPanic(t, "NewGauge_nil_callback", func() {
+		NewGauge("NewGauge_nil_callback", nil)
+	})
+	expectPanic(t, "GetOrCreateGauge_nil_callback", func() {
+		GetOrCreateGauge("GetOrCreateGauge_nil_callback", nil)
+	})
+}
+
 func TestGaugeSerial(t *testing.T) {
 	name := "GaugeSerial"
 	n := 1.23

metrics_test.go

@@ -14,6 +14,7 @@ func TestInvalidName(t *testing.T) {
 		expectPanic(t, fmt.Sprintf("NewGauge(%q)", name), func() { NewGauge(name, func() float64 { return 0 }) })
 		expectPanic(t, fmt.Sprintf("NewSummary(%q)", name), func() { NewSummary(name) })
 		expectPanic(t, fmt.Sprintf("GetOrCreateCounter(%q)", name), func() { GetOrCreateCounter(name) })
+		expectPanic(t, fmt.Sprintf("GetOrCreateGauge(%q)", name), func() { GetOrCreateGauge(name, func() float64 { return 0 }) })
 		expectPanic(t, fmt.Sprintf("GetOrCreateSummary(%q)", name), func() { GetOrCreateSummary(name) })
 	}
 	f("")
@@ -52,6 +53,12 @@ func TestGetOrCreateNotCounter(t *testing.T) {
 	expectPanic(t, name, func() { GetOrCreateCounter(name) })
 }
 
+func TestGetOrCreateNotGauge(t *testing.T) {
+	name := "GetOrCreateNotGauge"
+	NewCounter(name)
+	expectPanic(t, name, func() { GetOrCreateGauge(name, func() float64 { return 0 }) })
+}
+
 func TestGetOrCreateNotSummary(t *testing.T) {
 	name := "GetOrCreateNotSummary"
 	NewCounter(name)