message: test: GAE, GKE, GCF run on GCE
  Make tests ensure that getDefaultResource prefers GAE, GKE, GCF over
  GCE.
diff: @@ -186,7 +186,8 @@ describe('metadata', function() {
  metadata.logging.auth.getEnvironment = function(callback) {
  callback(null, {
  - IS_APP_ENGINE: true
  + IS_APP_ENGINE: true,
  + IS_COMPUTE_ENGINE: true
  });
  };
  @@ -209,7 +210,8 @@ describe('metadata', function() {
  metadata.logging.auth.getEnvironment = function(callback) {
  callback(null, {
  - IS_CLOUD_FUNCTION: true
  + IS_CLOUD_FUNCTION: true,
  + IS_COMPUTE_ENGINE: true
  });
  };
  @@ -255,6 +257,7 @@ describe('metadata', function() {
  metadata.logging.auth.getEnvironment = function(callback) {
  callback(null, {
  + IS_COMPUTE_ENGINE: true,
  IS_CONTAINER_ENGINE: true
  });
  };
  