fix(auth): support gRPC API keys

Porting googleapis/google-api-go-client#2326
which happened after we had made parity change to new auth lib.

auth/grpctransport/grpctransport.go

@@ -87,6 +87,9 @@ type Options struct {
 	// configured for the client, which will be compared to the universe domain
 	// that is separately configured for the credentials.
 	UniverseDomain string
+	// APIKey specifies an API key to be used as the basis for authentication.
+	// If set DetectOpts are ignored.
+	APIKey string
 
 	// InternalOptions are NOT meant to be set directly by consumers of this
 	// package, they should only be set by generated client code.
@@ -109,7 +112,8 @@ func (o *Options) validate() error {
 	if o.InternalOptions != nil && o.InternalOptions.SkipValidation {
 		return nil
 	}
-	hasCreds := o.Credentials != nil ||
+	hasCreds := o.APIKey != "" ||
+		o.Credentials != nil ||
 		(o.DetectOpts != nil && len(o.DetectOpts.CredentialsJSON) > 0) ||
 		(o.DetectOpts != nil && o.DetectOpts.CredentialsFile != "")
 	if o.DisableAuthentication && hasCreds {
@@ -237,8 +241,15 @@ func dial(ctx context.Context, secure bool, opts *Options) (*grpc.ClientConn, er
 		return nil, err
 	}
 
-	// Authentication can only be sent when communicating over a secure connection.
-	if !opts.DisableAuthentication {
+	if opts.APIKey != "" {
+		grpcOpts = append(grpcOpts,
+			grpc.WithPerRPCCredentials(&grpcKeyProvider{
+				apiKey:   opts.APIKey,
+				metadata: opts.Metadata,
+				secure:   secure,
+			}),
+		)
+	} else if !opts.DisableAuthentication {
 		metadata := opts.Metadata
 
 		var creds *auth.Credentials
@@ -283,6 +294,26 @@ func dial(ctx context.Context, secure bool, opts *Options) (*grpc.ClientConn, er
 	return grpc.DialContext(ctx, endpoint, grpcOpts...)
 }
 
+// grpcKeyProvider satisfies https://pkg.go.dev/google.golang.org/grpc/credentials#PerRPCCredentials.
+type grpcKeyProvider struct {
+	apiKey   string
+	metadata map[string]string
+	secure   bool
+}
+
+func (g *grpcKeyProvider) GetRequestMetadata(ctx context.Context, uri ...string) (map[string]string, error) {
+	metadata := make(map[string]string, len(g.metadata)+1)
+	metadata["X-goog-api-key"] = g.apiKey
+	for k, v := range g.metadata {
+		metadata[k] = v
+	}
+	return metadata, nil
+}
+
+func (g *grpcKeyProvider) RequireTransportSecurity() bool {
+	return g.secure
+}
+
 // grpcCredentialsProvider satisfies https://pkg.go.dev/google.golang.org/grpc/credentials#PerRPCCredentials.
 type grpcCredentialsProvider struct {
 	creds *auth.Credentials

auth/grpctransport/grpctransport_test.go

@@ -389,6 +389,28 @@ func TestNewClient_DetectedServiceAccount(t *testing.T) {
 	}
 }
 
+func TestGRPCKeyProvider_GetRequestMetadata(t *testing.T) {
+	apiKey := "MY_API_KEY"
+	reason := "MY_REQUEST_REASON"
+	ts := grpcKeyProvider{
+		apiKey: apiKey,
+		metadata: map[string]string{
+			"X-goog-request-reason": reason,
+		},
+	}
+	got, err := ts.GetRequestMetadata(context.Background())
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := map[string]string{
+		"X-goog-api-key":        ts.apiKey,
+		"X-goog-request-reason": reason,
+	}
+	if diff := cmp.Diff(want, got); diff != "" {
+		t.Errorf("mismatch (-want +got):\n%s", diff)
+	}
+}
+
 type staticTP struct {
 	tok *auth.Token
 }