* allow not to record metrics (response time, etc...)
* this commit doesn't change the UI. If the metrics are disabled
/stats and /stats/errors will return empty response.
in /preferences, the columns response time and reliability will be empty.
httpx.RequestError (subclass of httpx.HTTPError) has a property request.
This property raises a RuntimeError if the attributes _request is None.
To avoid a cascade of errors, this commit reads directly the _request attribute.
Some error won't stop the engine:
* additional HTTP redirects for example
* some invalid results
secondary=True allows to flag these errors as not important.
Alexandre Flament
Markus Heiser