update metric_desc doc to include metadata for instuctlab

[atheurer]

FYI, Updating an index template requires revving the CDM spec from "cdmv7dev" to "cdmv8dev", and also requires rickshaw project to start using cdmv8dev index names.

need to include "epoch" and "step" and "rank" to the metric_desc index template.

there might be more metadata, and we should probably wait a bit longer before we add these and require a rev (and rebuild) of the indices