Http_client oauth2 token refresh using refresh_token instead of client_credential

Hi all! I need to implement, using http_client, oauth2 authentication with refresh_token flow (instead client_credential).

Specific use case are reported inside Google AdWords docs:
https://developers.google.com/google-ads/api/rest/auth
this is what I need to archive.

I'm in difficult to find the place where I can put

  --data "grant_type=refresh_token" \
  --data "refresh_token=REFRESH_TOKEN" \

using output http_client of benthos because it seams that not manage this kind of two params for token refresh.

this is the error that I receive:

{"@timestamp":"2022-04-14T09:07:20Z","@service":"benthos","component":"benthos.output","level":"ERROR","message":"Failed to send message to http_client: https://googleads.googleapis.com/v10/customers/123-123-123:uploadClickConversions: Post "https://googleads.googleapis.com/v10/customers/123-123-123:uploadClickConversions/": oauth2: cannot fetch token: 400 Bad Request\nResponse: {\n "error": "unsupported_grant_type",\n "error_description": "Invalid grant_type: client_credentials"\n}"}

output:
  http_client:
    url: https://googleads.googleapis.com/v10/customers/123-123-1231:uploadClickConversions
    verb: POST
    headers:
      Content-Type: application/json
      developer-token: "DEVTOKEN"
    oauth2:
      enabled: true
      client_key: "CKEY"
      client_secret: "CSECRET"
      token_url: "https://www.googleapis.com/oauth2/v3/token" 

I'm in difficult to put

  --data "grant_type=refresh_token" \
  --data "refresh_token=REFRESH_TOKEN" \

somewhere :)

Start with the http_client output's oauth2 configuration and token_url handling described in the issue, then trace how the current client-credentials request is built. Confirm the expected Google Ads refresh-token parameters and define done as successfully obtaining and using a token with refresh_token authentication.