test(metrics): improve test coverage (#7538)

Add more unit tests for metrics plugin, around registration
deduplication, zone management, restart/shutdown behavior and
context helpers.

Increases test coverage from 54.8% to 76.1%.

Signed-off-by: Ville Vesilehto <ville@vesilehto.fi>

plugin/metrics/metrics_test.go

@@ -2,7 +2,9 @@ package metrics
 
 import (
 	"context"
+	"io"
 	"net"
+	"net/http"
 	"testing"
 	"time"
 
@@ -11,6 +13,8 @@ import (
 	"github.com/coredns/coredns/plugin/test"
 
 	"github.com/miekg/dns"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
 )
 
 func TestMetrics(t *testing.T) {
@@ -127,3 +131,87 @@ func TestMetricsHTTPTimeout(t *testing.T) {
 		t.Error("HTTP request did not time out")
 	}
 }
+
+func TestMustRegister_DuplicateOK(t *testing.T) {
+	met := New("localhost:0")
+	met.Reg = prometheus.NewRegistry()
+
+	g := promauto.NewGaugeVec(prometheus.GaugeOpts{Namespace: "test", Name: "dup"}, []string{"l"})
+	met.MustRegister(g)
+	// registering the same collector again should yield AlreadyRegisteredError internally and be ignored
+	met.MustRegister(g)
+}
+
+func TestRemoveZone(t *testing.T) {
+	met := New("localhost:0")
+
+	met.AddZone("example.org.")
+	met.AddZone("example.net.")
+	met.RemoveZone("example.net.")
+
+	zones := met.ZoneNames()
+	for _, z := range zones {
+		if z == "example.net." {
+			t.Fatalf("zone %q still present after RemoveZone", z)
+		}
+	}
+}
+
+func TestOnRestartStopsServer(t *testing.T) {
+	met := New("localhost:0")
+	if err := met.OnStartup(); err != nil {
+		t.Fatalf("startup failed: %v", err)
+	}
+
+	// server should respond before restart
+	resp, err := http.Get("http://" + ListenAddr + "/metrics")
+	if err != nil {
+		t.Fatalf("pre-restart GET failed: %v", err)
+	}
+	if resp != nil {
+		io.Copy(io.Discard, resp.Body)
+		resp.Body.Close()
+	}
+
+	if err := met.OnRestart(); err != nil {
+		t.Fatalf("restart failed: %v", err)
+	}
+
+	// after restart, the listener should be closed and request should fail
+	if _, err := http.Get("http://" + ListenAddr + "/metrics"); err == nil {
+		t.Fatalf("expected GET to fail after restart, but it succeeded")
+	}
+}
+
+func TestRegistryGetOrSet(t *testing.T) {
+	r := newReg()
+	addr := "localhost:12345"
+	pr1 := prometheus.NewRegistry()
+	got1 := r.getOrSet(addr, pr1)
+	if got1 != pr1 {
+		t.Fatalf("first getOrSet should return provided registry")
+	}
+
+	pr2 := prometheus.NewRegistry()
+	got2 := r.getOrSet(addr, pr2)
+	if got2 != pr1 {
+		t.Fatalf("second getOrSet should return original registry, got different one")
+	}
+}
+
+func TestOnRestartNoop(t *testing.T) {
+	met := New("localhost:0")
+	// without OnStartup, OnRestart should be a no-op
+	if err := met.OnRestart(); err != nil {
+		t.Fatalf("OnRestart returned error on no-op: %v", err)
+	}
+}
+
+func TestContextHelpersEmpty(t *testing.T) {
+	if got := WithServer(context.TODO()); got != "" {
+		t.Fatalf("WithServer(nil) = %q, want empty", got)
+	}
+	if got := WithView(context.TODO()); got != "" {
+		t.Fatalf("WithView(nil) = %q, want empty", got)
+	}
+}