feat(metrics): Unify datetime format

[mitsuhiko]

This is a preliminary work to later add an API that allows metrics emission as part of span creation. This somewhat unifies the APIs with regards to timestamps. The span system uses datetime objects, this now also permits these values in metrics and vice versa.

[antonpirker]

We need to change the end_timestamp in finish() to allow floats too, to be consistent

[antonpirker]

looks good