Add Job environment as labels

[igorbrites]

I wanted to create an alert every time a Job with the environment set to production fails, but I see the exporter doesn't add the environment as a label. Do you think adding this label makes sense, and do jobs without an environment return it empty or null?

Many thanks for considering my request.

[tboerger]

Generally it makes sense to add the label at least to the list of available labels, but we need to check what happens if no environment is defined.