Include database versions in ping data

Created by: tsenart

In order to confidently increase our Minimum Supported Version of Redis and Postgres, we need to know what our customers are actually relying on. Instead of sending them a survey, we can just gather this information automatically and include them in pings.

@flying-robot: Tentatively assigning to you for next cycle (starting this Wednesday), since you touched this area of the system recently and I think you could get this done easily. You can ready about the Life of a Ping for the full picture.

@ebrodymoore: Do we need to do anything beyond the code changes? (i.e. Looker, Dataflow schema, doc updates) I thought I saw a dev guide somewhere on how to add new data to pings, but can't find it.