Mirror the Tokens trend: add a clearable "Group by" select to the
MetricChartCard so the GPU Instances chart can split by instance type /
instance / user, and Storage by storage / user. When grouped, the chart
fetches group_by=["date", "<dim>"] (the same list style as the token usage
API) and pivots into one stacked series per group (shared buildTrendSeries
util), with a legend; ungrouped stays a single series. group_by is now a
list across the resource breakdown client; group-by options reuse the
bottom-table dimensions (Users only when org-wide).
