My question was about how you (think you are) setting log_error_verbosity, not what you're doing to produce the message. The decisions in elog.c about which fields to print in the server log do not depend on the latter.
Roger that.
However... looking again at what elog.c does, I'm prompted to ask whether you're using plain or CSV log output. The CSV log format doesn't consider log_error_verbosity when deciding whether to output this field.
Yup, we are using CSV log format. So in that case, is there no way around the context messages if I still want the log message?