Implement oauth2 using refresh_token

[Sindhura Gowri G S]

I have created an issue for Implementing oauth2 using refresh_token  - https://github.com/prometheus/prometheus/issues/10339

Proposal

Use case. Why is this important?

Current implementation of oauth2 uses client_credentials (client_id and client_secret) https://www.oauth.com/oauth2-servers/access-tokens/client-credentials/ but for oauth2 authentication using refresh_token requires (refresh_token secret) https://www.oauth.com/oauth2-servers/access-tokens/refreshing-access-tokens/.
When the endpoint has oauth2 authentication using refresh_token grant type, then we need to fetch access_token from user provided refresh_token secret and then hit the endpoint url.
Hence we shall add the support for oauth2 authentication using refresh_token along with client_credentials.

I am working on this enhancement and the proposed design is to introduce refresh_token and grant_type attributes in the oauth2 - https://prometheus.io/docs/prometheus/latest/configuration/configuration/#oauth2 and fetch access_token from given refresh_token or client_credentials.

Please confirm if I can proceed with this enhancement.

Thanks,

Sindhura