badboy changed the title Add support for the text metric type Bug 1726326 - Add support for the text metric type

This applies the following additional restrictions:
1) Metrics of this type will need to be data collection category 3 or 4.
2) Metrics of this type will only be allowed for custom pings. Sending a text metric in built-in pings is forbidden.
3) Metrics of this type will only be allowed to have a ping or application lifetime. Other lifetimes will be forbidden.
Restrictions are applied through the jsonschema.
This leads to okayish error messages for 1) and 3)
and a suboptimal, but understandable error message for 2).