Make it possible to pass functions to be run prior to flushing the response

[thekid]

This makes it easy to implement Server timing, which appears in the browser's developer console, as follows:

// Use performance.getEntriesByType('navigation')[0].serverTiming[0] to access via JS
// or check the browser's development console, network tab, "Timing" for a request
class WithServerTiming implements Filter {

  public function filter($req, $res, $invocation) {
    $start= microtime(true);
    $res->flushing(function($res) use($start) {
      $res->header('Server-Timing', sprintf('total;dur=%.3f', 1000 * (microtime(true) - $start)));
    });

    // This function will now run as soon as either a handler down the chain explicitely flushes the
    // response, or when this happens at the end of the request/response lifecycle in the server.
    return $invocation->proceed($req, $res);
  }
}

Ideally, we would do this via HTTP trailers which can be appended to the response, but this only works in Firefox, see https://www.fastly.com/blog/supercharging-server-timing-http-trailers/, so we need a way to append this information at the last moment possible: before finishing sending the headers. This is roughly the equivalent of https://github.com/jshttp/on-headers (see how NuxtJS does this) and intercepting Ktor's send pipeline, see https://ktor.io/docs/server-custom-plugins-base-api.html

---

For server metrics in ExpressJS, see https://www.npmjs.com/package/server-timing. They add a setMetric() method to response. While I don't like monkey-patching, I'd like to have an easy way of adding metrics. Maybe something like this?

// Inside above filter
$metrics= new Metrics();
$res->flushing(function($res) use($metrics) {
  foreach ($metrics as $name => $value) {
    $res->header('Server-Timing', sprintf('%s;dur=%.3f', $name, $value), true);
  }
});

return $invocation->proceed($metrics->attach($req), $res);

// Inside handler
function($req, $res) {
  $metrics= Metrics::for($req) ?? Metrics::$NULL;

  $metrics->start('db');
  $this->db->query(/* ... */);
  $metrics->stop('db');

  $res->send('OK', 'text/plain');
}

[thekid]

Question is whether we need to be able to hook into any other parts of the send pipeline, like writing, flushing or closing; or if we should leave that up to something wrapping around web.io.Output.