Move out from internal packages to allow import

[WindzCUHK]

Request

Move existing go files out from the internal package. Would it OK for us to make a PR for that?
https://github.com/enix/x509-certificate-exporter/tree/main/internal

Problem

Hi, we are using the x509-certificate-exporter internally and it works great. Thank you very much for your work.
Recently, we have a use case to export more metrics from our application (certificate read frequency, certificate update frequency, etc.). As we want to limit the number of exported ports due to internal policy, we are planning to integrate the x509-certificate-exporter as a go library inside our application to report certificate metrics and the extra metrics using the same exporter port.

However, as the current metric definitions (collector.go) and the related go files are located in the golang's internal package, we cannot import the x509-certificate-exporter in our application and run the exporter server internally. We would like to make PR to move out from the internal package. Please let us know if you have any other suggestions using this project as a library or idea on the new package name.