Error prometheus metrics

[chelomontilla]

Hi,
It would be great to have a prometheus metric with the number of errors, it would be helpful to detect possible issues with some images.
What do you think?

Thanks

[davidcollom]

@MPV / @chelomontilla - I agree, this could be a great addition to the project

[maria-reynoso]

This will be added in the next release with PR #345