A system for alerting based on time-series data, with its own rule language. The language (along with many others) required authors have demonstrated they can adhere to the style guide by going through a process to obtain readability.
It's a language and supporting infrastructure for collecting and querying time series data for monitoring.
It was replaced a long time ago by a new system called monarch, but a few holdouts will probably continue using borgmon until the heat death of the universe.
If monarch supported large customers it would take over those holdouts
prometheus 0.1
This is the correct answer.
To elaborate on this answer: Prometheus was written by ex-Googlers who had no shame in copying Borgmon pretty much exactly, syntax-compatible and all.