After the transition period we have to deprecate the ability to pass arbitrary strings to TRACE_EVENTx macros and enforce the use of structured protos.
This is to audit PII in events and avoid that somebody can leak data via a string without us noticing. 
All events with potential PII (read: stuff that contains arbitrary strings) should have its own proto, get a review, and be flagged as safe/unsafe.

In the meantime we'll proceed with the TV1 approach, where we allow strings in events but discard them for the use case of slow reports.