Prometheus uses GitHub to manage reviews of pull requests.
If you have a trivial fix or improvement, go ahead and create a pull request, addressing (with @...) the maintainer of this repository (see MAINTAINERS.md) in the description of the pull request.
If you plan to do something more involved, first discuss your ideas on our mailing list. This will avoid unnecessary work and surely give you and us a good deal of inspiration.
Relevant coding style guidelines are the Go Code Review Comments and the Formatting and style section of Peter Bourgon's Go: Best Practices for Production Environments.