Add ability for policies to define required metadata fields

[ezekg]

Would be helpful for certain workflows that use metadata for e.g. billing purposes where licenses are created by sales/success teams, i.e. not created as part of a script or automation. Maybe something like this would work, using JSON pointers? Alternative would be to use an object, but I don't really want to make this too complex e.g. with type assertions, etc.

{
  "requireMetadata": ["/foo", "/bar/baz", "/qux/0/quxx"]
}