When no grouping dimensions are provided, the default behavior for time-series data is to group on time-series (_tsid) and list all dimensions in the output. This is needed for parity with promql, as well as for discovering available dimensions per time series.

TS metrics | STATS rate(http_reqs) BY TBUCKET(5m)
TS metrics | STATS rate(http_reqs) BY TBUCKET(5m), TDIM_ALL()