Add error class as label for sidekiq_job_failed

[leklund]

It might be useful to have the error class added as a label when a job fails.

[DuoSRX]

That would definitely be very useful! The only thing is this, from the Prometheus docs:

CAUTION: Remember that every unique combination of key-value label pairs represents a new time series, which can dramatically increase the amount of data stored. Do not use labels to store dimensions with high cardinality (many different label values), such as user IDs, email addresses, or other unbounded sets of values.

I don't think we would have that many different errors anyway but it's something to keep an eye on.

[leklund]

Perhaps it could be a configurable option so that users could turn off the additional labeling if they expect to have many different label values for the error classes.

[DuoSRX]

Yeah. Sounds good to me 👍