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/setup_test.go

@@ -40,3 +40,10 @@ func TestPrometheusParse(t *testing.T) {
 		}
 	}
 }
+
+func TestSetupBasic(t *testing.T) {
+	c := caddy.NewTestController("dns", "prometheus localhost:9153")
+	if err := setup(c); err != nil {
+		t.Fatalf("setup returned error: %v", err)
+	}
+}