external-dns: Log messages showing as errors in Google Stackdriver

I would like to reopen issue #772 because our Stackdriver logs are spammed with errors that are actually external-dns logs with level=info. There are a couple ways we can fix that:

1. Provide a configuration option to write log severity in severity JSON attributes instead of level. Eg. by setting --log-format=stackdriver_json.

2. Provide a configuration option to write all debug and info logs to stdout, and only rest to stderr. Or list of severities that would be sent to stdout.

I think the first option is preferred because then we can use severity filters in Stackdriver. Please note that there is no way how we can fix Stackdriver configuration to recognize info level because it’s Fluentd agent configuration is managed by GKE team 😕.