http2: log debug only when in debug mode

PR-URL: #43626
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ricky Zhou <0x19951125@gmail.com>

lib/internal/http2/core.js

@@ -190,10 +190,10 @@ let debug = require('internal/util/debuglog').debuglog('http2', (fn) => {
   debug = fn;
 });
 
-// TODO(addaleax): See if this can be made more efficient by figuring out
-// whether debugging is enabled before we perform any further steps. Currently,
-// this seems pretty fast, though.
 function debugStream(id, sessionType, message, ...args) {
+  if (!debug.enabled) {
+    return;
+  }
   debug('Http2Stream %s [Http2Session %s]: ' + message,
         id, sessionName(sessionType), ...new SafeArrayIterator(args));
 }