Be able to monitor a list of Nginx instances

[TomClqncy]

Is your feature request related to a problem? Please describe.
Today, the exporter can only monitor a single instance of Nginx. In the case of 1000s of instances, you would need 1000s of exporter instances. Painful ;)

Describe the solution you'd like
Specify a list of Nginx instances to monitor so the exporter can monitor all of them and publish the metrics including the instance names

Describe alternatives you've considered
Running 1000s of exporters, not an easy and cost effective solution

Additional context
Large Kubernetes deployment with thousands of APis with a Nginx instance for each

[nginx-bot]

Hi @TomClqncy! Welcome to the project! 🎉

Thanks for opening this issue!
Be sure to check out our Contributing Guidelines and the Issue Lifecycle while you wait for someone on the team to take a look at this.

[TomClqncy]

#539 (comment)