feat(metricsserver): add Gatherer method

Add explicit Gatherer() method to improve API discoverability
and prevent users from accidentally using prometheus.DefaultGatherer
instead of the custom registry.

Changes:
- Add Gatherer() method returning prometheus.Gatherer interface
- Add NewWithDefaultGatherer() constructor for opt-in default usage
- Update package docs with usage examples
- Add tests for both gatherer modes

metricsserver/metrics_test.go

@@ -67,6 +67,47 @@ func TestRegistry(t *testing.T) {
 	}
 }
 
+func TestGatherer(t *testing.T) {
+	metrics := New()
+
+	gatherer := metrics.Gatherer()
+	if gatherer == nil {
+		t.Fatal("Gatherer() returned nil")
+	}
+
+	// Register a test metric
+	counter := prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "test_gatherer_counter",
+		Help: "A test counter for gatherer",
+	})
+
+	metrics.Registry().MustRegister(counter)
+	counter.Inc()
+
+	// Test that the gatherer collects our custom metric
+	metricFamilies, err := gatherer.Gather()
+	if err != nil {
+		t.Errorf("failed to gather metrics: %v", err)
+	}
+
+	found := false
+	for _, mf := range metricFamilies {
+		if mf.GetName() == "test_gatherer_counter" {
+			found = true
+			break
+		}
+	}
+
+	if !found {
+		t.Error("registered metric not found via Gatherer()")
+	}
+
+	// Verify gatherer is the same as registry
+	if gatherer != metrics.r {
+		t.Error("Gatherer() should return the same object as the registry for custom registry mode")
+	}
+}
+
 func TestHandler(t *testing.T) {
 	metrics := New()
 
@@ -212,6 +253,45 @@ func TestListenAndServeContextCancellation(t *testing.T) {
 	}
 }
 
+func TestNewWithDefaultGatherer(t *testing.T) {
+	metrics := NewWithDefaultGatherer()
+
+	if metrics == nil {
+		t.Fatal("NewWithDefaultGatherer returned nil")
+	}
+
+	if !metrics.useDefaultGatherer {
+		t.Error("useDefaultGatherer should be true")
+	}
+
+	gatherer := metrics.Gatherer()
+	if gatherer == nil {
+		t.Fatal("Gatherer() returned nil")
+	}
+
+	// Verify it returns the default gatherer
+	if gatherer != prometheus.DefaultGatherer {
+		t.Error("Gatherer() should return prometheus.DefaultGatherer when useDefaultGatherer is true")
+	}
+
+	// Verify the custom registry is still available and separate
+	if metrics.Registry() == nil {
+		t.Error("Registry() should still return a custom registry")
+	}
+
+	// Test that registering in custom registry doesn't affect default gatherer check
+	counter := prometheus.NewCounter(prometheus.CounterOpts{
+		Name: "test_default_gatherer_counter",
+		Help: "A test counter",
+	})
+	metrics.Registry().MustRegister(counter)
+
+	// The gatherer should still be the default one, not our custom registry
+	if metrics.Gatherer() != prometheus.DefaultGatherer {
+		t.Error("Gatherer() should continue to return prometheus.DefaultGatherer")
+	}
+}
+
 // Benchmark the metrics handler response time
 func BenchmarkMetricsHandler(b *testing.B) {
 	metrics := New()