fix: GoogleAdsError missing using generator version after 1.3.0 (#89)

[PR](googleapis/gapic-generator-typescript#878) within
updated gapic-generator-typescript version 1.4.0

Committer: @summer-ji-eng
PiperOrigin-RevId: 375759421

Source-Link: googleapis/googleapis@95fa72f

Source-Link: https://github.com/googleapis/googleapis-gen/commit/f40a34377ad488a7c2bc3992b3c8d5faf5a15c46

src/v1/connection_service_client.ts

@@ -140,6 +140,8 @@ export class ConnectionServiceClient {
     }
     if (!opts.fallback) {
       clientHeader.push(`grpc/${this._gaxGrpc.grpcVersion}`);
+    } else if (opts.fallback === 'rest') {
+      clientHeader.push(`rest/${this._gaxGrpc.grpcVersion}`);
     }
     if (opts.libName && opts.libVersion) {
       clientHeader.push(`${opts.libName}/${opts.libVersion}`);