feat(metrics): add OTLP metrics support with centralized config

- Create new metrics/ package for OpenTelemetry-native metrics with OTLP export
- Refactor OTLP configuration to internal/tracerconfig/ to eliminate code duplication
- Add consistent retry configuration across all HTTP OTLP exporters
- Add configuration validation and improved error messages
- Include test coverage for all new functionality
- Make OpenTelemetry metrics dependencies explicit in go.mod

Designed for new applications requiring structured metrics export to
observability backends via OTLP protocol.

metricsserver/metrics_test.go

@@ -15,11 +15,11 @@ import (
 
 func TestNew(t *testing.T) {
 	metrics := New()
-	
+
 	if metrics == nil {
 		t.Fatal("New returned nil")
 	}
-	
+
 	if metrics.r == nil {
 		t.Error("metrics registry is nil")
 	}
@@ -28,32 +28,32 @@ func TestNew(t *testing.T) {
 func TestRegistry(t *testing.T) {
 	metrics := New()
 	registry := metrics.Registry()
-	
+
 	if registry == nil {
 		t.Fatal("Registry() returned nil")
 	}
-	
+
 	if registry != metrics.r {
 		t.Error("Registry() did not return the metrics registry")
 	}
-	
+
 	// Test that we can register a metric
 	counter := prometheus.NewCounter(prometheus.CounterOpts{
 		Name: "test_counter",
 		Help: "A test counter",
 	})
-	
+
 	err := registry.Register(counter)
 	if err != nil {
 		t.Errorf("failed to register metric: %v", err)
 	}
-	
+
 	// Test that the metric is registered
 	metricFamilies, err := registry.Gather()
 	if err != nil {
 		t.Errorf("failed to gather metrics: %v", err)
 	}
-	
+
 	found := false
 	for _, mf := range metricFamilies {
 		if mf.GetName() == "test_counter" {
@@ -61,7 +61,7 @@ func TestRegistry(t *testing.T) {
 			break
 		}
 	}
-	
+
 	if !found {
 		t.Error("registered metric not found in registry")
 	}
@@ -69,7 +69,7 @@ func TestRegistry(t *testing.T) {
 
 func TestHandler(t *testing.T) {
 	metrics := New()
-	
+
 	// Register a test metric
 	counter := prometheus.NewCounterVec(
 		prometheus.CounterOpts{
@@ -80,40 +80,40 @@ func TestHandler(t *testing.T) {
 	)
 	metrics.Registry().MustRegister(counter)
 	counter.WithLabelValues("GET").Inc()
-	
+
 	// Test the handler
 	handler := metrics.Handler()
 	if handler == nil {
 		t.Fatal("Handler() returned nil")
 	}
-	
+
 	// Create a test request
 	req := httptest.NewRequest("GET", "/metrics", nil)
 	recorder := httptest.NewRecorder()
-	
+
 	// Call the handler
 	handler.ServeHTTP(recorder, req)
-	
+
 	// Check response
 	resp := recorder.Result()
 	defer resp.Body.Close()
-	
+
 	if resp.StatusCode != http.StatusOK {
 		t.Errorf("expected status 200, got %d", resp.StatusCode)
 	}
-	
+
 	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		t.Fatalf("failed to read response body: %v", err)
 	}
-	
+
 	bodyStr := string(body)
-	
+
 	// Check for our test metric
 	if !strings.Contains(bodyStr, "test_requests_total") {
 		t.Error("test metric not found in metrics output")
 	}
-	
+
 	// Check for OpenMetrics format indicators
 	if !strings.Contains(bodyStr, "# TYPE") {
 		t.Error("metrics output missing TYPE comments")
@@ -122,7 +122,7 @@ func TestHandler(t *testing.T) {
 
 func TestListenAndServe(t *testing.T) {
 	metrics := New()
-	
+
 	// Register a test metric
 	counter := prometheus.NewCounterVec(
 		prometheus.CounterOpts{
@@ -133,46 +133,46 @@ func TestListenAndServe(t *testing.T) {
 	)
 	metrics.Registry().MustRegister(counter)
 	counter.WithLabelValues("GET").Inc()
-	
+
 	ctx, cancel := context.WithCancel(context.Background())
 	defer cancel()
-	
+
 	// Start server in a goroutine
 	errCh := make(chan error, 1)
 	go func() {
 		// Use a high port number to avoid conflicts
 		errCh <- metrics.ListenAndServe(ctx, 9999)
 	}()
-	
+
 	// Give the server a moment to start
 	time.Sleep(100 * time.Millisecond)
-	
+
 	// Test metrics endpoint
 	resp, err := http.Get("http://localhost:9999/metrics")
 	if err != nil {
 		t.Fatalf("failed to GET /metrics: %v", err)
 	}
 	defer resp.Body.Close()
-	
+
 	if resp.StatusCode != http.StatusOK {
 		t.Errorf("expected status 200, got %d", resp.StatusCode)
 	}
-	
+
 	body, err := io.ReadAll(resp.Body)
 	if err != nil {
 		t.Fatalf("failed to read response body: %v", err)
 	}
-	
+
 	bodyStr := string(body)
-	
+
 	// Check for our test metric
 	if !strings.Contains(bodyStr, "test_requests_total") {
 		t.Error("test metric not found in metrics output")
 	}
-	
+
 	// Cancel context to stop server
 	cancel()
-	
+
 	// Wait for server to stop
 	select {
 	case err := <-errCh:
@@ -186,21 +186,21 @@ func TestListenAndServe(t *testing.T) {
 
 func TestListenAndServeContextCancellation(t *testing.T) {
 	metrics := New()
-	
+
 	ctx, cancel := context.WithCancel(context.Background())
-	
+
 	// Start server
 	errCh := make(chan error, 1)
 	go func() {
 		errCh <- metrics.ListenAndServe(ctx, 9998)
 	}()
-	
+
 	// Give server time to start
 	time.Sleep(100 * time.Millisecond)
-	
+
 	// Cancel context
 	cancel()
-	
+
 	// Server should stop gracefully
 	select {
 	case err := <-errCh:
@@ -215,7 +215,7 @@ func TestListenAndServeContextCancellation(t *testing.T) {
 // Benchmark the metrics handler response time
 func BenchmarkMetricsHandler(b *testing.B) {
 	metrics := New()
-	
+
 	// Register some test metrics
 	for i := 0; i < 10; i++ {
 		counter := prometheus.NewCounter(prometheus.CounterOpts{
@@ -225,18 +225,18 @@ func BenchmarkMetricsHandler(b *testing.B) {
 		metrics.Registry().MustRegister(counter)
 		counter.Add(float64(i * 100))
 	}
-	
+
 	handler := metrics.Handler()
-	
+
 	b.ResetTimer()
-	
+
 	for i := 0; i < b.N; i++ {
 		req := httptest.NewRequest("GET", "/metrics", nil)
 		recorder := httptest.NewRecorder()
 		handler.ServeHTTP(recorder, req)
-		
+
 		if recorder.Code != http.StatusOK {
 			b.Fatalf("unexpected status code: %d", recorder.Code)
 		}
 	}
-}
+}