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 nginxinc/nginx-plus-go-client#222

[alitoufighi]

My assumption is we won't be able to fix this with the given solution for nginxinc/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 🙂

[alitoufighi]

Hmm maybe beginning the connection in a minimal way (is finding the lowest compatible api version any easier? i.e. 1?) and then promoting it to a higher version? (I mean in the client)

Also I don't think the call to get API versions is over a specific API version, so it could be doable to communicate over to find the max compatible API version during initialization as well (in the nginx client repo, could be a breaking change?). It might not be the cleanest way either. (might actually give this a shot)

[mpstefan]

Blocked until nginx-plus-go-client 1.3.0 is released. When it is, @alitoufighi would you be open to creating a PR for this?