Allow using the latest version with older versions of NGINX Plus

[lucacome]

I want to be able to use the exporter with older versions of NGINX Plus and not have the latest version of the API hardcoded.

Depends on nginx/nginx-plus-go-client#222

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[alitoufighi]

My assumption is we won't be able to fix this with the given solution for nginx/nginx-plus-go-client#222 as it needs an already connected client.

[lucacome]

@alitoufighi yes we need a connection to get the API versions from NGINX. One of the options that I was thinking about was to check the version every time we collect the metrics, but that might be a little bit expensive.

I'm open to suggestions 🙂