benchmark: add implicit-framing case to http/set-header

The existing `setHeader` and `setHeaderWH` cases set `Content-Length`
explicitly, and `normal` passes every header to `writeHead()`. The very
common handler shape `res.setHeader(...)` + `res.end(body)`, where
`end()` derives the framing, exercises a different path and was not
covered.

Signed-off-by: Matteo Collina <hello@matteocollina.com>
PR-URL: https://github.com/nodejs/node/pull/66420
Reviewed-By: Tim Perry <pimterry@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

benchmark/fixtures/simple-http-server.js

@@ -114,6 +114,12 @@ module.exports = http.createServer((req, res) => {
       else
         res.writeHead(status, { 'Content-Length': len.toString() });
       break;
+    case 'setHeaderImplicit':
+      // Let end() derive the message framing: no Content-Length or
+      // Transfer-Encoding header and no writeHead() call.
+      res.statusCode = status;
+      res.setHeader('Content-Type', 'text/plain');
+      break;
     default:
       if (chunkedEnc) {
         res.writeHead(status, {

benchmark/http/set-header.js

@@ -2,7 +2,7 @@
 const common = require('../common.js');
 
 const bench = common.createBenchmark(main, {
-  res: ['normal', 'setHeader', 'setHeaderWH'],
+  res: ['normal', 'setHeader', 'setHeaderWH', 'setHeaderImplicit'],
   duration: 5,
 });
 
@@ -15,6 +15,7 @@ const c = 50;
 // normal: writeHead(status, {...})
 // setHeader: statusCode = status, setHeader(...) x2
 // setHeaderWH: setHeader(...), writeHead(status, ...)
+// setHeaderImplicit: statusCode = status, setHeader(...) x1, framing left to end()
 function main({ res, duration }) {
   const server = require('../fixtures/simple-http-server.js')
   .listen(0)