Move GA keys to enum

[eonist]

Suggestion:

extension Telemetry {
   enum Key {
      enum ScreenView: String {
         case dh, dp, dt, pageview
      }
      enum Session: String {
         case sc, dp, start, end, session
      }
      enum Exception: String {
         case exd, exf, exception
      }
      enum Timing: String {
         case utc, utv, utl, utt, timing
      }
      enum Event: String {
         case ec, ea, el, event
      }
      enum Meta: String {
         case tid, aid, cid, an, av, ua, ul, sr, v
      }
      enum Other: String {
         case aip, t
      }
   }

Also add descriptions and comments etc?