create `first_complete_date` and add to metrics-state json object

[sidetrackedmind]

There's a last_complete_date key in a json object in S3 - s3://opentransit-pdx/metrics-state/v1/metrics-state_v1_trimet.json.
We need to create a first_complete_date key and store in the same json. Then the banner would grab these dates and display them.

[youngj]

It would also be useful to know the range of dates where scheduled metrics are available. Users may want to view dates in the app where scheduled metrics are available even if the observed metrics are not available.