Allow google client.Logger options.

[krasi-georgiev]

gc := &zapgcl.Core{
		Logger:          client.Logger(gclLogID, opts),
		SeverityMapping: zapgcl.DefaultSeverityMapping,
}

The use case is that want to record the hostname when sending logs to google cloud, but don't need it when looking at the logs locally.

With this change can use the CommonLabels option to set custom labels only for the google client.

func CommonLabels(m map[string]string) LoggerOption { return commonLabels(m) }
```