Make the collector crate public

[popadi]

Any chance to make the collector sub-module public? I'm interested in using the collect method to retrieve an instance of a Metrics struct and then expose it.

Why? My app is using prometheus/client_rust instead of metrics-exporter-prometheus, so the metrics export needs to be a bit different (more info about the Collector trait here, if you are interested). I could benefit from reusing some of the structs and functions defined in that sub-module.