(feature) Expose flag_name label on all_flag_evaluations_total

[MattIPv4]

Requirements

👋 As discussed in Slack, it would be useful to me if there were a metric exposed that tracked the bulk evaluations for each flag. While this would essentially expose the current flag config as a metric, I believe it may be useful to myself and others to be able to see at any given point in time, via the metrics, which flags are being sent to clients during bulk evaluations.

This will be rather noisy though, massively increasing the cardinality of all_flag_evaluations_total, so my suggestion as part of this would be to expose a new flag/setting that enables the additional flag_name label on all_flag_evaluations_total, so folks can choose to opt-in to this additional cardinality if they so desire, or to keep the current single-counter functionality of this metric.

[thomaspoignant]

For the context, the slack discussion is here.