Command to write to the error log

It would be helpful to have a command to instruct the server to write an arbitrary log message to the error log.

A well-running database may not log any lines to the error log during normal operations, but it's impossible to know for certain if the database has any issues writing to the log file (for example, a log rotation system gone wrong).

Having a server command be able to write a log line, like a heart beat message, would allow us to confirm the system capable of writing to the error log, and thus, the server is running fine.