Add time-unit awareness to timespan metrics

[edugfilho]

Following 1630966 GLAM needs to be aware of the time-unit of a timespan type metric (Glean).
This will allow the timespans to change their time-unit (from milliseconds to seconds, for example) without causing aggregation problems.
Here's how I see this being implemented in the ETL and application without having to add new columns:'

• Talk with the Glean team to gather the finest time-unit (microseconds, for example) that a metric can assume.
• During the ETL, transform all timespan values to that time-unit and aggregate them.
• Once in the app, fetch the timespan metric current time-unit and transform the values to it.

This work will likely require data to be migrated

[badboy]

I guess the probeinfo history for a metric has the time_unit for every history entry, so one could pick the first