ref(dashboards): Simplify TimeSeries field meta

[gggritso]

There is an incoming re-write of the /events-stats/ endpoint, which will include a new and much nicer data output format. This PR start to align with that format with one very nice simplification, of what the meta field looks like. Since TimeSeries is already a transformed type (we make it from EventsStats, usually) we can make changes to it in preparation for the API to return TimeSeries straight, eventually. Plus these changes are good and useful, and make the code simpler.

Contributes to https://github.com/getsentry/team-performance/issues/58

Before:

{
  "meta": {
    "fields": {
      "eps()": "rate"
    },
    "units": {
      "eps()": "1/second"
    }
  }
}

This format is very awkward, because you have to go digging through the meta to find the type and unit. This can be very annoying if you don't know what the field was, because that information wasn't passed along. This is even more annoying because in TimeSeries, if its ever renamed, we have to also rename the fields in the meta, so they match the field. Yikes.

After:

{
  "meta": {
    "type": "rate",
    "unit": "1/second"
  }
}

This is much more sensible. No need to go digging for types and units, no need to rename anything. Since almost all the code is going through convertSeriesToTimeSeries I didn't even have to change much, mostly mocks and stories.