Refactor exporter so that it can be used as a library.

[tamalsaha]

We are using the Postgres exported in our project as a library. This pr refactors the exporter to a collector subpackage so that it can be used as a library.

[coveralls]

Coverage increased (+2.8%) to 57.399% when pulling d07635c on kubedb:master into 3e6cf08 on wrouesnel:master.

[wrouesnel]

I like this commit. I'll refactor it in the next few days to fit the new repository structure just rolled out, but great work!

[wrouesnel]

Okay so looking into this further, I'm merging the overall idea but it's very conditional - it's a supremely hacky way to use it as a library and should be regarded as unstable. The interface is going to be aggressively changed in the near future to factor out all the global variables which make it pretty unsafe to use as is.

[tamalsaha]

@wrouesnel, sounds good to me with your caveat. For now we are vendoring. Looking forward to stable state.

[hbagdi]

@wrouesnel friendly ping for any update on this.

[wrouesnel]

Working on it. There's a fair bit of refactoring to make it sensible (and to support multiple exporter instances sanely).

I'm also pulling the built-in queries out to a YAML file format so everything can be overridden.

[wrouesnel]

What am I saying? This Github! - the work libraryize work is happening on this branch here: https://github.com/wrouesnel/postgres_exporter/tree/libraryize_mk2

[tamalsaha]

@wrouesnel, wondering what are the plans on this one.

[hbagdi]

@wrouesnel any update on this? This would be an awesome feature to monitor multiple Postgres instances using a single exporter. Appreciate your efforts, thank you!