fix: enable keepAlive for HTTP requests

gax/src/fallbackServiceStub.ts

@@ -18,18 +18,42 @@
 /* global AbortController */
 
 import nodeFetch from 'node-fetch';
-import {Response as NodeFetchResponse} from 'node-fetch';
+import {Response as NodeFetchResponse, RequestInit} from 'node-fetch';
 import {AbortController as NodeAbortController} from 'abort-controller';
 
-import {hasWindowFetch, hasAbortController} from './featureDetection';
+import {hasWindowFetch, hasAbortController, isNodeJS} from './featureDetection';
 import {AuthClient} from './fallback';
 import {StreamArrayParser} from './streamArrayParser';
 import {pipeline, PipelineSource} from 'stream';
+import type {Agent as HttpAgent} from 'http';
+import type {Agent as HttpsAgent} from 'https';
 
 interface NodeFetchType {
   (url: RequestInfo, init?: RequestInit): Promise<Response>;
 }
 
+// Node.js before v19 does not enable keepalive by default.
+// We'll try to enable it very carefully to make sure we don't break possible non-Node use cases.
+// TODO: remove this after Node 18 is EOL.
+// More info: https://github.com/node-fetch/node-fetch#custom-agent
+let agentOption:
+  | ((parsedUrl: {protocol: string}) => HttpAgent | HttpsAgent)
+  | null = null;
+let httpAgent: HttpAgent | null = null;
+let httpsAgent: HttpsAgent | null = null;
+if (isNodeJS()) {
+  const http = require('http');
+  httpAgent = new http.Agent({keepAlive: true});
+  const https = require('https');
+  httpsAgent = new https.Agent({keepAlive: true});
+  agentOption = (parsedUrl: {protocol: string}) => {
+    if (parsedUrl.protocol === 'http:') {
+      return httpAgent!;
+    }
+    return httpsAgent!;
+  };
+}
+
 export interface FallbackServiceStub {
   // Compatible with gRPC service stub
   [method: string]: (
@@ -141,13 +165,16 @@ export function generateServiceStub(
             method: fetchParameters.method,
             signal: cancelSignal,
           };
+          if (agentOption) {
+            fetchRequest.agent = agentOption;
+          }
           if (
             fetchParameters.method === 'GET' ||
             fetchParameters.method === 'DELETE'
           ) {
             delete fetchRequest['body'];
           }
-          return fetch(url, fetchRequest);
+          return fetch(url, fetchRequest as {});
         })
         .then((response: Response | NodeFetchResponse) => {
           if (response.ok && rpc.responseStream) {