Update dependency google.golang.org/api to v0.15.1

Steps:
./hack/pin-dependency.sh google.golang.org/api v0.15.1
./hack/update-vendor.sh
./hack/verify-vendor.sh
./hack/pin-dependency.sh github.com/googleapis/gax-go/v2 v2.0.5
./hack/update-vendor.sh
./hack/verify-vendor.sh
./hack/lint-dependencies.sh

vendor/google.golang.org/api/internal/settings.go

@@ -1,16 +1,6 @@
-// Copyright 2017 Google LLC
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//      http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
+// Copyright 2017 Google LLC.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
 
 // Package internal supports the options and transport packages.
 package internal
@@ -27,19 +17,20 @@ import (
 // DialSettings holds information needed to establish a connection with a
 // Google API service.
 type DialSettings struct {
-	Endpoint        string
-	Scopes          []string
-	TokenSource     oauth2.TokenSource
-	Credentials     *google.Credentials
-	CredentialsFile string // if set, Token Source is ignored.
-	CredentialsJSON []byte
-	UserAgent       string
-	APIKey          string
-	Audiences       []string
-	HTTPClient      *http.Client
-	GRPCDialOpts    []grpc.DialOption
-	GRPCConn        *grpc.ClientConn
-	NoAuth          bool
+	Endpoint          string
+	Scopes            []string
+	TokenSource       oauth2.TokenSource
+	Credentials       *google.Credentials
+	CredentialsFile   string // if set, Token Source is ignored.
+	CredentialsJSON   []byte
+	UserAgent         string
+	APIKey            string
+	Audiences         []string
+	HTTPClient        *http.Client
+	GRPCDialOpts      []grpc.DialOption
+	GRPCConn          *grpc.ClientConn
+	NoAuth            bool
+	TelemetryDisabled bool
 
 	// Google API system parameters. For more information please read:
 	// https://cloud.google.com/apis/docs/system-parameters