Add support for multiple scrape addresses

[brandon1024]

Proposed changes

Make the nginx.scrape-uri accept a slice of addresses, and register prometheus collectors for each monitored nginx instance.

This revision came out of a discussion in #229. The general concensus by those in the thread was that it would be better to run an nginx-exporter for every monitored nginx instance. I had implemented these changes for my own use, so I decided to make them available to the wider community here.

Feel free to merge, or don't :-) I'm fine with either.

Checklist

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• I have read the CONTRIBUTING
  guide
• I have proven my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have ensured the README is up to date
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• I will ensure my PR is targeting the main branch and pulling from my branch on my own fork

[lucacome]

Hi @brandon1024, thanks for the PR!

It seems like a relatively small change and it would make a few people happy 🙂

My only concern is that we're adding a new label addr to all the metrics and this is generally not recommended (as I mentioned in the issue). Do you think it would make sense to add the label only if there are multiple scrape URIs? So the majority of the users would not see any difference when running the exporter with just one URI? Or do we want to keep it consistent?

Also, do you think this is considered a breaking change?

[brandon1024]

Thanks for the review @lucacome 🙂

My only concern is that we're adding a new label addr to all the metrics and this is generally not recommended (as I mentioned in the issue). Do you think it would make sense to add the label only if there are multiple scrape URIs? So the majority of the users would not see any difference when running the exporter with just one URI? Or do we want to keep it consistent?

That's fair, and this did cross my mind when I was implementing these changes. I agree it would be better to add the address label only when more than one address is given. This will be pretty easy to implement, I'll make some tweaks today and push a new revision.

Also, do you think this is considered a breaking change?

Once I make those minor changes, it would be completely backwards compatible :-)

[brandon1024]

Done!

[lucacome]

Thanks @brandon1024 !