How to obtain an oauth access token to use the Google BigQuery Api

[ken-slabs]

I would like to expose a BigQuery stored procedure that returns data back to the caller based on parameters provided through an Apigee proxy.  As I understand, an oAuth access token is required in order to call the Google BigQuery API.

I found an article that explains using a predefined service account to deploy the proxy with the necessary BigQuery permissions.  Basically, it explained about using a ServiceCallOut policy to retrieve the access token but I am not sure how to properly setup the code in the policy to obtain the access token.  Or is it the correct approach (I included the snippet in the ServiceCallOut policy below)

https://www.googlecloudcommunity.com/gc/Apigee/How-to-get-Bearer-token-for-Management-APIs-in-Apigee...

https://cloud.google.com/apigee/docs/api-platform/security/google-auth/overview

ServiceCallOut code

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>

<ServiceCallout continueOnError="false" enabled="true" name="SC-GetOAuthAccessToken">

  <DisplayName>SC-GetOAuthAccessToken</DisplayName>

  <Properties/>

  <Request clearPayload="true" variable="myRequest">

    <IgnoreUnresolvedVariables>false</IgnoreUnresolvedVariables>

  </Request>

  <Response>calloutResponse</Response>

  <HTTPTargetConnection>

    <Properties/>

    <URL>https://accounts.google.com/o/oauth2/v2/auth</URL>

    <Authentication>

      <GoogleAccessToken>

        <Scopes>

          <Scope>https://www.googleapis.com/auth/bigquery https://www.googleapis.com/auth/cloud-platform https://www.googleapis.com/auth/cloud-platform.read-only</Scope>

        </Scopes>

      </GoogleAccessToken>

    </Authentication>

  </HTTPTargetConnection>

</ServiceCallout>