While in a regular case a specific metric for a specific host will be
provider by a single instance (exporter) so aggregating by label and by
intances should be the same, it is more correct to aggregate by the same
label that the one we use to filter the metrics.
This is follow up of https://review.opendev.org/c/openstack/watcher/+/944795
Related-Bug: #2103451
Change-Id: Ia61f051547ddc51e0d1ccd5a56485ab49ce84c2e