exporter: missing github_repo_release_downloads metrics #52

Hey! 👋 I was trying out this project and don't see github_repo_release_downloads being exported. I'm not sure if this is due to the updated API for releases / downloads or something else.

I'm running this image:

Image ID:       docker-pullable://infinityworks/github-exporter@sha256:d89edcc0cd16310984ac8afbf5a32be0067b2aa76a972fff5c07ab5800fb6dd9