docs: example Prometheus alerts for return ratios

[adamdecaf]

PayGate Version: v0.9.0-dev

What were you trying to do? When running PayGate according to NACHA rules there needs to be monitoring on returned entries in order to stay within the NACHA guidelines. Violations can cause suspension of an ACH subscription into the network.

What did you expect to see? Our documentation and guides should offer production grade alerting.

What did you see?

The Dwolla blog has a good summary on these return ratios. They're pulled from the NACHA guidelines as well.

• Administrative Returns must stay below three percent. This percentage is calculated based on ACH debit returns for the preceding 60 days on the following return reason codes: R02, R03 and R04.
• Unauthorized Returns must stay below 0.5 percent. This percentage is calculated based on ACH debit returns for the preceding 60 days on the following return reason codes: R05, R07, R10, R29 and R51.
• Overall Returns must stay below 15 percent. This percentage is calculated based on ACH debit returns for the preceding 60 days and includes all return reason codes.
• NSF Returns have no specific threshold by themselves; however, these returns contribute to the Overall Return rate (see above) and make up the largest number of returns, by volume. NSF Returns include R01 and R09.

[adamdecaf]

We should add some prometheus metrics around returned entries including their code. Right now we've just got metrics around the returned files, which is very different from these metrics/alerts.