Annotations and Prometheus: Showing “current time” line

[jescombe]

• What Grafana version and what operating system are you using?
  v9.5.1 docker image on CentOS

• What are you trying to achieve?
  Am trying to add the current time as an annotation line on a time series chart spanning now-12h to now+12h.

• How are you trying to achieve it?
  Similar to this post, but am using a Prometheus data source for this panel (the back end is actually VictoriaMetrics).
  I have been attempting to make a comparable annotation for the Prometheus data source, but have not been able to find a syntax that gives me a single annotation line.

• What happened?
  For instance, I though that tlast_change_over_time(mqtt_value[1m]) would return a timestamp for the most recently updated mqqt_value metric, but what I get is an annotation line every 15mins up to the current time.
  

  
  
  Thanks in advance for any pointers!