Improve grouping of user creation errors in New Relic monitoring

[robertknight]

Errors from the user creation API are not being grouped together properly in New Relic. This is because the error messages include usernames and user IDs which vary with each message, but don't match any of NR's automated rules for ignoring values.

When a customer inadvertently triggered a large number of these errors, this made it difficult to tell NR to ignore them.

Slack thread: https://hypothes-is.slack.com/archives/C074BUPEG/p1716891448114809

[robertknight]

From the current error messages, it is useful to be able to see what authority (and therefore which service or customer) the user accounts belong to.

One way that we could resolve this would be to use the API in the Python NR agent that allows us to specify a custom grouping key for exceptions: https://docs.newrelic.com/docs/apm/agents/python-agent/python-agent-api/seterrrorgroupcallback-python-agent-api/