web/outpost: make metrics compliant with Prometheus best-practices
Today, all NewHistogramVec store values in nanoseconds without changing
the default histogram bucket, which are made for seconds, making them
a bit useless. In addition, some metrics names are not self-explanatoryand
and do not comply with Prometheus best practices.
This commit tries to fix all of this "issues".
NOTE: I kept old metrics in order to avoid breaking changes with
existing dashboards and metrics.
Signed-off-by: Alexandre NICOLAIE <xunleii@users.noreply.github.com>
