chore(http): Clarify Trailer support, partial support for Server-Timing in Fx devtools only

[bsmth]

Summary

The support for HTTP Trailer header is misleading, as it's removed from the Fetch spec & not accessible there anyway / XHR. As far as I can see, the only support that exists in browsers is exclusively for Server-Timing, and at that, it's only used for DevTools in Network -> Timing, all other headers (as trailers) are a no-op / discarded. For this reason, I've set it to partial support for Trailer, and full support for Server-Timing header as trailer, which no other browser appears to do.

Test results and supporting details

• Firefox Server-Timing implementation bug -> https://bugzilla.mozilla.org/show_bug.cgi?id=1403051:

  • Networking aspect landed in Fx59 -> https://bugzilla.mozilla.org/show_bug.cgi?id=1413999
  • Exposed in DevTools in Fx71 -> https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/71#developer_tools
  • MDN docs issue -> [DevTools] Network Monitor, Fx 71 tasks sprints#2277
  • Necko source for handling of trailers

• ChrBugs -> https://issues.chromium.org/issues/40811358

Node repro:

const http = require("http");
http
  .createServer((req, res) => {
    res.writeHead(200, {
      "Content-Type": "text/plain",
      "Transfer-Encoding": "chunked",
      // Server-Timing as header
      // "Server-Timing": "custom-metric;dur=567.123",
      Trailer: "Server-Timing",
    });
    res.write("Hello.\n");
    // Server-Timing as trailer
    // See https://nodejs.org/api/http.html#responseaddtrailersheaders
    res.addTrailers({
      "Server-Timing": "custom-metric;dur=666.123",
    });
    res.end();
  })
  .listen(3000, () => {
    console.log("Server running at http://localhost:3000");
  });

Related issues

• http.headers.Trailer - Chromium-based browsers do not have "full support" for the Trailer header #24684
• http.headers.Trailer - Not meaningfully supported in Chrome #18441
• http.headers.Trailer - reading trailers not supported by Fetch API #14703
• Issue with "Trailer": page says all browsers are compatible but there is no way to use this feature content#7137

[caugner]

FWIW I only found this indirectly related WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=178069

[caugner]

LGTM.

Compact diff grouped by file:

[caugner]

One nit.

[caugner]

Oh, we should probably add the Chromium impl_url here:

https://issues.chromium.org/issues/40811358