feat: Add support for fetching config from env

[abhisek]

I am following the official documentation to setup Grafana with datasource-syncer for Managed Prometheus: https://cloud.google.com/stackdriver/docs/managed-prometheus/query#use-kubernetes

Our standard approach for providing config & secrets are through environment. Adding environment as a fallback source for config when not provided through command line

[bernot-dev]

This seems like a reasonable feature to support.

I'm not sure we would just want to support providing just a subset of the flags though. Any particular rationale for those three?

Also, the commit message lister will require a lowercase "A".

Thanks for the PR!

[abhisek]

This seems like a reasonable feature to support.

Thanks. This will help us setup the datasource-syncer within our k8s while maintaining single source of truth for config (i.e. our env providers).

I'm not sure we would just want to support providing just a subset of the flags though. Any particular rationale for those three?

Specifically for our use, the CA/Key file flags are not required from environment because we can make them available as volume mounts with any file path. A k8s secret is mounted as a file and the file name is a local concern (i.e. within the job yaml) hence the actual path does not matter and can hardcoded as a command line argument.

However I have added env fallback for ca_file, cert_file, key_file for consistency in user experience. I have consciously decided not to add a fallback for gcmEndpointOverride because I feel the defaults are appropriate. In rare cases where it needs to be overridden I think we should force its explicit usage through command line .

Also, the commit message lister will require a lowercase "A".

Fixed.

Thanks for the PR!