title: 403 error when trying to list metrics in different project through monitoring api from app engine
body: <p>I am using an app engine application written in python2.7 to collect monitoring metrics from different projects. I am running this from cloud shell  The app engine application is deployed in projectA and it is able to collect the metrics from projectA through the below call</p>
  <pre><code>service = build('monitoring', 'v3', cache_discovery=True)
      project_name = 'projects/{project_id}'.format(
          project_id=project_id
      )
  
      metrics = service.projects().metricDescriptors().list(
           name=project_name,
           pageSize=config.PAGE_SIZE,
           pageToken=next_page_token
      ).execute()
  </code></pre>
  <p>This call is written in a loop, Now i need to collect ProjectB metrics, I have owner access to ProjectB. When ProjectB is passed as parameter to project_id I am getting this below error:</p>
  <pre><code>                        logMessage: &quot;Error: &lt;HttpError 403 when requesting https://monitoring.googleapis.com/v3/projects/ProjectB/metricDescriptors?pageToken=&amp;alt=json&amp;pageSize=500 returned &quot;Permission monitoring.metricDescriptors.list denied (or the resource may not exist).&quot;&gt;&quot;     
  severity: &quot;ERROR&quot;     
  sourceLocation: {
   file: &quot;/base/data/home/apps/s~ProjectA/list-metrics:20200706t123743.427891295940019389/main.py&quot;      
   functionName: &quot;post&quot;      
   line: &quot;665&quot;      
  }
  time: &quot;2020-07-06T16:10:43.724399Z&quot;
  </code></pre>
  <p>I am not sure what should be done to make this work?
  I am very new to google cloud and its apis and also new to app engine,python2.7 kindly help, Thanks</p>