inject credentials from env vars

[mazay]

Thanks for all the work on this, I've just deployed it and I'm really happy about the amount of available metrics and overall functionality.

I've used the k8s manifests from the repo to deploy the app and they worked just fine, however, having the entire config in a secret is suboptimal.

It would be great if we could inject credentials from environment variables or some separate config file, most of us probably would have shared monitoring credentials across all the devices so it would be more secure and handy.