Interesting, but way harder than it sounds. Very often log systems live as a service for multiple microservices that do real world work. Propagating sentinel data through each of those systems is a nightmare because:

- those systems often have real-world secondary effects

- they sometimes have a tendency to validate away sentinel data prior to logging because the data is no good (e.g. a credit card number that isn't real or a password for a user that doesn't exist), although that can depend on scale and cost

- cross-team coordination of how to handle the sentinel data introduces coupling across teams/services which is contrary to the goal of microservices.