Other components, such as `vmagent`, mark these flags as sensitive and
hide them from the `/metrics` endpoint by default. This commit adds
similar handling to the `vmalert` component, hiding them by default, to
prevent logging of secrets inappropriately.
Showing of these values is controlled by an additional flag.
Follow up to https://github.com/VictoriaMetrics/VictoriaMetrics/pull/2947
* vmalert: allow to disable automatically added path to remote write address via disablePathAppend flag
* docs: update docs to include remoteWrite.disablePathAppend
The change adds no new functionality and aims to move flags definitions
to subpackages that are using them. This should improve readability
of the main function.