Please review core identifiers patch https://gerrit.wikimedia.org/r/615559

Specific things to look for / think about:

• clarity of field descriptions, if there's any ambiguity
• whether it's okay for this schema fragment to reference /fragments/analytics/common fragment and declare client_dt as required (the intention is to avoid referencing both every time, since any schema that would be using this fragment will be using client_dt anyway)

Note: T259714 is blocked until the identifiers fragment is available for use in schemas