When does it not fit?
Prometheus values reliability. You can always view what statistics are
available about your system, even under failure conditions. If you need 100%
accuracy, such as for per-request billing, Prometheus is not a good choice as
the collected data will likely not be detailed and complete enough. In such a
case you would be best off using some other system to collect and analyze the
data for billing, and Prometheus for the rest of your monitoring.