Should both of these be using the same prefix?  Having TASK_COUNTER and TASK_ATTEMPT share seems okay, provided that TASK_COUNTER is only stored in a task row, and TASK_ATTEMPT is only reported/stored in a task attempt row, ie. we don't have collisions for the same row.

Are these different from task-level counters in Hadoop 1.0?  Those use COUNTER_COLUMN_PREFIX currently ("g").  If these are the same this we should stick with the current format.