CaptureMetrics reports 200 after a zero-byte ReadFrom #42

With httpsnoop v1.0.4, this handler sends HTTP 502 to the client, but CaptureMetrics reports status code 200:

  http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
      _, err := io.Copy(w, iotest.ErrReader(errors.New("upstream failed")))
      if err != nil {
          http.Error(w, err.Error(), http.StatusBadGateway)
      }
  })

(Wrap it with CaptureMetrics and serve through a net/http server for a repro)

The ReadFrom hook sets headerWritten=true even when zero bytes were copied:

Consequently, e.g. a subsequent 5xx response emission or a 204 response emission would be tracked with a status code of 200 (which is the initial value, see here).

As a solution, we could set headerWritten only when n > 0,

We probably want regression tests for two cases, for example:

• source returns an error before producing any bytes; the handler then sends 502, the client receives it, and httpsnoop captures the 502
• source reaches EOF without producing any bytes; the handler then sends 204, the client receives it, and httpsnoop captures the 204

Related: #32, #38