prometheus-adapter: add prefix option to config for container metrics (#1844)

This commit adds the options `containerMetricsPrefix`
to the prometheus-adapter config-map generator. By default this option
is the empty string and doesn't change the current behavior. If set
however to e.g. `pa_`, the prometheus-adapter configuration will add
this prefix to all container_ queries in the resource rules.
This enables users of kube-prometheus to define a specialised service
monitor, that only expose the prometheus-adapter related container metrics with a
different configuration, like `honorTimestamps: true` or a tighter
scrape interval.

Signed-off-by: Jan Fajerski <jfajersk@redhat.com>