[ttml2] optionality of metric component in offset time expression

In https://github.com/w3c/ttml2/issues/561, we removed optionality of the
unit component of a length expression. However, we have not done the same
with the metric component of offset time expressions, which now (in TTML2)
allows for the defaulting of the metric (to seconds).

Do we still want the time metric to be optional? Has anyone implemented
this (besides TTT tools)? Are authors using this yet?

I'm asking because we either need to remove optionality (reverting to TTML1
syntax) or add a new feature in order to address
https://github.com/w3c/ttml2/issues/752.