fix: Add Content-Type header to the health endpoint (#2634)

* fix: Content-Type and Grpc-Metadata-Content-Type headers with the health endpoint #2611

* fix: no need to set grpc content type header

* fix: remove unnecessary grpc header comment

* test: lighten the Content-Type header test

runtime/mux.go

@@ -229,6 +229,8 @@ func WithHealthEndpointAt(healthCheckClient grpc_health_v1.HealthClient, endpoin
 					return
 				}
 
+				w.Header().Set("Content-Type", "application/json")
+
 				if resp.GetStatus() != grpc_health_v1.HealthCheckResponse_SERVING {
 					var err error
 					switch resp.GetStatus() {

runtime/mux_test.go

@@ -719,6 +719,26 @@ func TestWithHealthzEndpoint_serviceParam(t *testing.T) {
 	}
 }
 
+func TestWithHealthzEndpoint_header(t *testing.T) {
+	for _, tt := range healthCheckTests {
+		t.Run(tt.name, func(t *testing.T) {
+			mux := runtime.NewServeMux(runtime.WithHealthzEndpoint(&dummyHealthCheckClient{status: tt.status, code: tt.code}))
+
+			r := httptest.NewRequest(http.MethodGet, "/healthz", nil)
+			rr := httptest.NewRecorder()
+
+			mux.ServeHTTP(rr, r)
+
+			if actualHeader := rr.Header().Get("Content-Type"); actualHeader != "application/json" {
+				t.Errorf(
+					"result http header Content-Type for grpc code %q and status %q should be application/json, got %s",
+					tt.code, tt.status, actualHeader,
+				)
+			}
+		})
+	}
+}
+
 var _ grpc_health_v1.HealthClient = (*dummyHealthCheckClient)(nil)
 
 type dummyHealthCheckClient struct {