AdmissionControlTimeSinceLastUpdate TIME_MS counter breaks some profile consumers

The issue is:

• type is a required field for TCounter with enum type TUnit
• Some readers (e.g. Java) that encounter an unknown enum value replace it it with null, then throw an exception because it is a required field

This means that the workflow for adding a new TUnit is basically broken, since we can't add a unit in Impala then update readers later.

I think we should switch to TIME_NS then reconsider the workflow here.