This patch adds rclone_http_status_code counter vector labeled by
* host,
* method,
* code.
It allows to see HTTP errors, backoffs etc.
The Metrics struct is designed for extensibility.
Adding new metrics is a matter of adding them to Metrics struct and including them in the response handling.
This feature has been discussed in the forum [1].
[1] https://forum.rclone.org/t/prometheus-metrics/14484
