promauto: make it available to avoid reflect when building labels

Signed-off-by: Eugene <eugene@amberpixels.io>

prometheus/promsafe/safe.go

@@ -43,6 +43,13 @@ func SetupGlobalPromauto(factoryArg ...promauto.Factory) {
 	}
 }
 
+// CustomLabelsProvider is an interface that allows to convert anything to a prometheus.Labels
+// It allows to provide your own FAST implementation of Struct->prometheus.Labels conversion
+// without using reflection.
+type CustomLabelsProvider interface {
+	ToPrometheusLabels() prometheus.Labels
+}
+
 // promsafeTag is the tag name used for promsafe labels inside structs.
 // The tag is optional, as if not present, field is used with snake_cased FieldName.
 // It's useful to use a tag when you want to override the default naming or exclude a field from the metric.
@@ -273,6 +280,10 @@ func extractLabelsWithValues(labelProvider labelsProviderMarker) prometheus.Labe
 		return nil
 	}
 
+	if clp, ok := labelProvider.(CustomLabelsProvider); ok {
+		return clp.ToPrometheusLabels()
+	}
+
 	// TODO: let's handle defaults as well, why not?
 
 	// Here, then, it can be only a struct, that is a parent of StructLabelProvider
@@ -291,13 +302,20 @@ func extractLabelValues(labelProvider labelsProviderMarker) []string {
 }
 
 // extractLabelNames extracts labels names from a given labelsProviderMarker (parent instance of aStructLabelProvider)
+// Deprecated: refactor is required. Order of result slice is not guaranteed.
 func extractLabelNames(labelProvider labelsProviderMarker) []string {
 	if any(labelProvider) == nil {
 		return nil
 	}
 
+	var labels prometheus.Labels
+	if clp, ok := labelProvider.(CustomLabelsProvider); ok {
+		labels = clp.ToPrometheusLabels()
+	} else {
+		labels = extractLabelFromStruct(labelProvider)
+	}
+
 	// Here, then, it can be only a struct, that is a parent of StructLabelProvider
-	labels := extractLabelFromStruct(labelProvider)
 	labelNames := make([]string, 0, len(labels))
 	for k := range labels {
 		labelNames = append(labelNames, k)

prometheus/promsafe/safe_test.go

@@ -21,6 +21,10 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promsafe"
 )
 
+// Important: This is not a test file. These are only examples!
+//            These can be considered smoke tests, but nothing more.
+//            TODO: Write real tests
+
 func ExampleNewCounterVecT_multiple_labels_manual() {
 	// Manually registering with multiple labels
 
@@ -140,6 +144,35 @@ func ExampleNewCounterVecT_pointer_to_labels_promauto() {
 	// Output:
 }
 
+// FastMyLabels is a struct that will have a custom method that converts to prometheus.Labels
+type FastMyLabels struct {
+	promsafe.StructLabelProvider
+	EventType string
+	Source    string
+}
+
+// ToPrometheusLabels does a fast conversion to labels. No reflection involved.
+func (f FastMyLabels) ToPrometheusLabels() prometheus.Labels {
+	return prometheus.Labels{"event_type": f.EventType, "source": f.Source}
+}
+
+func ExampleNewCounterVecT_fast_safe_labels_provider() {
+	// It's possible to use pointer to labels struct
+	myReg := prometheus.NewRegistry()
+
+	counterOpts := prometheus.CounterOpts{
+		Name: "items_counted_fast",
+	}
+
+	c := promsafe.WithAuto[FastMyLabels](myReg).NewCounterVecT(counterOpts)
+
+	c.With(FastMyLabels{
+		EventType: "reservation", Source: "source1",
+	}).Inc()
+
+	// Output:
+}
+
 func ExampleNewCounterVecT_single_label_manual() {
 	// Manually registering with a single label
 	// Example of usage of shorthand: no structs no generics, but one string only