feat: Split PDM metric panel for gprd and gstg

feat: Split PDM metric panel for gprd and gstg

Related to gitlab-com/gl-infra/delivery#21042 (closed) and gitlab-com/gl-infra/delivery#21193 (closed)

Instead of a single panel with both numbers, I have split it into two panels. This is useful because now, we will see an environment-wise message when the metric could not be found. For instance, it is possible that during a selected time period, there is a metric for gstg but no metric for gprd. In this case, the gstg panel will show the metric while the gprd panel will ask the user to select a longer time period. This is not possible if both numbers are displayed in the same panel.

Test dashboard: https://dashboards.gitlab.net/goto/PfI_-DrHR?orgId=1

Screenshots

Before	After
Notice that both numbers are shown in the same panel.	Notice that there are two separate panels showing these numbers now.

When there is no metric, this is what the dashboard will look like:

No metric from gstg or gprd	Metric exists from gstg but not from gprd

The case where a metric exists from gprd but non exists from gstg is unlikely, because we always deploy to gstg first.