perf_hooks: import http2 only once

Import http2 internal bindings on each collectHttp2Stats call is not
required.

PR-URL: #29419
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>

lib/perf_hooks.js

@@ -92,11 +92,12 @@ const IDX_SESSION_STATS_DATA_SENT = 6;
 const IDX_SESSION_STATS_DATA_RECEIVED = 7;
 const IDX_SESSION_STATS_MAX_CONCURRENT_STREAMS = 8;
 
+let http2;
 let sessionStats;
 let streamStats;
 
 function collectHttp2Stats(entry) {
-  const http2 = internalBinding('http2');
+  if (http2 === undefined) http2 = internalBinding('http2');
   switch (entry.name) {
     case 'Http2Stream':
       if (streamStats === undefined)