Prometheus Query Language (w. Grafana expresssion)

Arithmetic operation safe

scrape_duration_seconds + on(device) group_left() avg(android_device_name) by (device)

group_left: Left side is fine, but right side has extra labels? Use group_left()!
group_left: group_left() does NOT "group" values— it just keeps??? extra labels on the right
on(): on() selects which labels must match. group_left() allows extra ones to stay

Muliply by 1, but add extra label (from right)

avg(android_scrape_duration_seconds)without(_dummy) * on(instance) group_left(secureBT)android_device_name
  Same:
avg(android_scrape_duration_seconds)without(_dummy) * on(instance) group_left(secureBT)android_device_name{}
  Grafana:
avg(android_scrape_duration_seconds)without(_dummy) * on(instance) group_left(secureBT)android_device_name{secureBT=~"$gBT"}