0.12.0: Optional error class label in failed jobs counter

Added

• Optional capture of error class for failed jobs counter.

  Set label_for_error_class_on_sidekiq_jobs_failed to true to add error label to sidekiq_jobs_failed_total metric.

  Pull request #34 by [@niborg]

Changed

• Stop including development-related files into packaged gem to avoid confusing users or software tools. [@Envek]