feat: Add new CollectorFunc utility

prometheus/collectorfunc.go

@@ -0,0 +1,30 @@
+// Copyright 2025 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus
+
+// CollectorFunc is a convenient way to implement a Prometheus Collector
+// without interface boilerplate.
+// This implementation is based on DescribeByCollect method.
+// familiarize yourself to it before using.
+type CollectorFunc func(chan<- Metric)
+
+// Collect calls the defined CollectorFunc function with the provided Metrics channel
+func (f CollectorFunc) Collect(ch chan<- Metric) {
+	f(ch)
+}
+
+// Describe sends the descriptor information using DescribeByCollect
+func (f CollectorFunc) Describe(ch chan<- *Desc) {
+	DescribeByCollect(f, ch)
+}

prometheus/collectorfunc_test.go

@@ -0,0 +1,83 @@
+// Copyright 2025 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package prometheus
+
+import "testing"
+
+func TestCollectorFunc(t *testing.T) {
+	testDesc := NewDesc(
+		"test_metric",
+		"A test metric",
+		nil, nil,
+	)
+
+	cf := CollectorFunc(func(ch chan<- Metric) {
+		ch <- MustNewConstMetric(
+			testDesc,
+			GaugeValue,
+			42.0,
+		)
+	})
+
+	ch := make(chan Metric, 1)
+	cf.Collect(ch)
+	close(ch)
+
+	metric := <-ch
+	if metric == nil {
+		t.Fatal("Expected metric, got nil")
+	}
+
+	descCh := make(chan *Desc, 1)
+	cf.Describe(descCh)
+	close(descCh)
+
+	desc := <-descCh
+	if desc == nil {
+		t.Fatal("Expected desc, got nil")
+	}
+
+	if desc.String() != testDesc.String() {
+		t.Fatalf("Expected %s, got %s", testDesc.String(), desc.String())
+	}
+}
+
+func TestCollectorFuncWithRegistry(t *testing.T) {
+	reg := NewPedanticRegistry()
+
+	cf := CollectorFunc(func(ch chan<- Metric) {
+		ch <- MustNewConstMetric(
+			NewDesc(
+				"test_metric",
+				"A test metric",
+				nil, nil,
+			),
+			GaugeValue,
+			42.0,
+		)
+	})
+
+	if err := reg.Register(cf); err != nil {
+		t.Errorf("Failed to register CollectorFunc: %v", err)
+	}
+
+	collectedMetrics, err := reg.Gather()
+	if err != nil {
+		t.Errorf("Failed to gather metrics: %v", err)
+	}
+
+	if len(collectedMetrics) != 1 {
+		t.Errorf("Expected 1 metric family, got %d", len(collectedMetrics))
+	}
+}

prometheus/examples_test.go

@@ -736,3 +736,51 @@ func ExampleNewConstMetricWithCreatedTimestamp() {
 	// Output:
 	// {"counter":{"value":1257894000,"createdTimestamp":"1970-01-01T00:00:00Z"}}
 }
+
+// Using CollectorFunc that registers the metric info for the HTTP requests.
+func ExampleCollectorFunc() {
+	desc := prometheus.NewDesc(
+		"http_requests_info",
+		"Information about the received HTTP requests.",
+		[]string{"code", "method"},
+		nil,
+	)
+
+	// Example 1: 42 GET requests with 200 OK status code.
+	collector := prometheus.CollectorFunc(func(ch chan<- prometheus.Metric) {
+		ch <- prometheus.MustNewConstMetric(
+			desc,
+			prometheus.CounterValue, // Metric type: Counter
+			42,                      // Value
+			"200",                   // Label value: HTTP status code
+			"GET",                   // Label value: HTTP method
+		)
+
+		// Example 2: 15 POST requests with 404 Not Found status code.
+		ch <- prometheus.MustNewConstMetric(
+			desc,
+			prometheus.CounterValue,
+			15,
+			"404",
+			"POST",
+		)
+	})
+
+	prometheus.MustRegister(collector)
+
+	// Just for demonstration, let's check the state of the metric by registering
+	// it with a custom registry and then let it collect the metrics.
+
+	reg := prometheus.NewRegistry()
+	reg.MustRegister(collector)
+
+	metricFamilies, err := reg.Gather()
+	if err != nil || len(metricFamilies) != 1 {
+		panic("unexpected behavior of custom test registry")
+	}
+
+	fmt.Println(toNormalizedJSON(sanitizeMetricFamily(metricFamilies[0])))
+
+	// Output:
+	// {"name":"http_requests_info","help":"Information about the received HTTP requests.","type":"COUNTER","metric":[{"label":[{"name":"code","value":"200"},{"name":"method","value":"GET"}],"counter":{"value":42}},{"label":[{"name":"code","value":"404"},{"name":"method","value":"POST"}],"counter":{"value":15}}]}
+}