Update Migration Detail dashboard counts

What does this MR do and why

This MR updates a few metrics in the Migration Detail dashboard that are showing some bobbling due to inconsistencies or lag between the different data sources. The update smoothes out the data to show counts that accurately reflect the production data. Looking at the sources, we see the zfs shard is the source that does not have the correct count, so by targeting default, we are able to exclude the inaccurate data.

Test dashboard: https://dashboards.gitlab.net/dashboard/snapshot/iWAcJQaveN61guizIoIxSWrXYBMWFjvs

Before	After