metics: allow maps in metrics structs

This allows us to have a map of metrics in a struct.

Epic: none
Release note: None

pkg/util/metric/registry.go

@@ -123,6 +123,8 @@ func (r *Registry) AddMetricStruct(metricStruct interface{}) {
 	}
 	t := v.Type()
 
+	const allowNil = true
+	const disallowNil = false
 	for i := 0; i < v.NumField(); i++ {
 		vfield, tfield := v.Field(i), t.Field(i)
 		tname := tfield.Name
@@ -137,14 +139,18 @@ func (r *Registry) AddMetricStruct(metricStruct interface{}) {
 			for i := 0; i < vfield.Len(); i++ {
 				velem := vfield.Index(i)
 				telemName := fmt.Sprintf("%s[%d]", tname, i)
-				// Permit elements in the array to be nil.
-				const skipNil = true
-				r.addMetricValue(ctx, velem, telemName, skipNil, t)
+				r.addMetricValue(ctx, velem, telemName, allowNil, t)
+			}
+		case reflect.Map:
+			iter := vfield.MapRange()
+			for iter.Next() {
+				// telemName is only used for assertion errors.
+				telemName := iter.Key().String()
+				r.addMetricValue(ctx, iter.Value(), telemName, allowNil, t)
 			}
 		default:
 			// No metric fields should be nil.
-			const skipNil = false
-			r.addMetricValue(ctx, vfield, tname, skipNil, t)
+			r.addMetricValue(ctx, vfield, tname, disallowNil, t)
 		}
 	}
 }
@@ -274,7 +280,7 @@ func checkFieldCanBeSkipped(
 	}
 
 	switch fieldType.Kind() {
-	case reflect.Array, reflect.Slice:
+	case reflect.Array, reflect.Slice, reflect.Map:
 		checkFieldCanBeSkipped(skipReason, fieldName, fieldType.Elem(), parentType)
 	case reflect.Struct:
 		containsMetrics := false
@@ -317,7 +323,7 @@ func containsMetricType(ft reflect.Type) bool {
 	}
 
 	switch ft.Kind() {
-	case reflect.Slice, reflect.Array:
+	case reflect.Slice, reflect.Array, reflect.Map:
 		return containsMetricType(ft.Elem())
 	case reflect.Struct:
 		for i := 0; i < ft.NumField(); i++ {

pkg/util/metric/registry_test.go

@@ -97,6 +97,7 @@ func TestRegistry(t *testing.T) {
 		StructHistogram     IHistogram
 		NestedStructGauge   NestedStruct
 		ArrayStructCounters [4]*Counter
+		MapOfCounters       map[int32]*Counter
 		// Ensure that nil struct values in arrays are safe.
 		NestedStructArray [2]*NestedStruct
 		// A few extra ones: either not exported, or not metric objects.
@@ -125,6 +126,10 @@ func TestRegistry(t *testing.T) {
 			nil, // skipped
 			NewCounter(Metadata{Name: "array.struct.counter.3"}),
 		},
+		MapOfCounters: map[int32]*Counter{
+			1: NewCounter(Metadata{Name: "map.counter.0"}),
+			2: NewCounter(Metadata{Name: "map.counter.1"}),
+		},
 		NestedStructArray: [2]*NestedStruct{
 			0: nil, // skipped
 			1: {
@@ -154,6 +159,8 @@ func TestRegistry(t *testing.T) {
 		"array.struct.counter.0":      {},
 		"array.struct.counter.1":      {},
 		"array.struct.counter.3":      {},
+		"map.counter.0":               {},
+		"map.counter.1":               {},
 		"nested.struct.array.1.gauge": {},
 	}
 	totalMetrics := len(expNames)