Adwords controls metrics for a normal adwords account.
for adwords, first setup a MCC/adwords manager account -- https://adwords.google.com/home/tools/manager-accounts/
Add the client account in this one using https://support.google.com/adwords/answer/6139186?hl=en#link
From the MCC account, create a developer token and apply for a basic token...its easy...not much required...
Now, for the development part
Use this library: https://github.com/googleads/google-api-ads-ruby/tree/master/adwords_api
Use this wiki: https://github.com/googleads/google-api-ads-ruby/wiki/API-access-using-own-credentials-%28installed-application-flow%29
Create OAuth2 credentials for the client account(NOT THE MCC ACCOUNT) using https://developers.google.com/adwords/api/docs/guides/authentication#installed
and running setup_auth2.rb
Run and get results.

For testing:
Testing cannot be done on production client
So, create a test MCC account using your production MCC account: 
https://developers.google.com/adwords/api/docs/guides/accounts-overview#getting_started_with_test_accounts
Step 1: Create a new gmail account for test mcc account and using the above link, create test MCC account.
Step 2: Using your production manager account, you must first create a test manager account prior to creating test client accounts. 
Step 3: Create client accounts under the test manager account and they will be automatically marked as test accounts.
Step 4: Use the developer token generated in non test MCC account to query
Step 5: Ensure you are logged in as the test MCC account(not client account) when generating oauth2 credentials using developer console.

For production:
Run setup_oauth2.rb again using the oauth2 credentials for production client account and you are good to go.