Charts by epoch

[Panaetius]

Currently, all charts in the dashboard use wallclock for the x-axis.

Using epoch is often more valuable, especially when comparing different numbers of workers.

We currently save epoch information in the 'metadata' field of logged metrics, it'd make sense to add epoch as a top-level, optional field (float) on the kubemetrics database table for this, for perfomance/sanity reasons. So this should be done as well before adding charts based on epoch