Adhere to telemetry schema 0.0.2

* The schema and version of telemetry data is exposed through the
  Content-Type header instead of through a custom HTTP Header.

See [Prometheus Client Data Exposition Format][1] for more details.

[1]: https://docs.google.com/a/soundcloud.com/document/d/1ZjyKiKxZV83VI9ZKAXRGKaUKK2BIWCT7oiGBKDBpjEY/edit#heading=h.wnviarbnyxcj

This is dependent on prometheus/prometheus#192