* refactor: create log.Infof and log.Warnf
* refactor: review DNS providers.
- use one `http.Client` by provider instead of one client by request
- use the same receiver name `d` for all `DNSProvider`
- use `http.MethodXXX`
* refactor: logger init.
This commit fixes the naming inconsistency of Google Cloud DNS being
named `googlecloud` in the source and `gcloud` in the user interface.
Furthermore, improving my previous commit, I realized that the project ID
is stored in the service account file. We can save one environment variable
when using service account file (e.g. in automation applications).