Launch exporter with collect-metrics-time

The existing dashboards make use of a metric called `scrape_duration_seconds`. I think the equivalent metric in openstack-exporter is `openstack_metric_collect_seconds`. This is only collected if `--collect-metric-time` is passed to openstack-exporter.

If we want to use this metric, we must enable this option for the exporter service.

Start by locating how the openstack-exporter service is configured and where its command-line arguments are defined. Enable --collect-metric-time for the exporter service, then verify that the openstack_metric_collect_seconds metric is collected and available for the existing dashboards.

Written by the indexing model from the issue text.