403 error when trying to list metrics in different project through monitoring api from app engine

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

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()

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:

                        logMessage: "Error: <HttpError 403 when requesting https://monitoring.googleapis.com/v3/projects/ProjectB/metricDescriptors?pageToken=&alt=json&pageSize=500 returned "Permission monitoring.metricDescriptors.list denied (or the resource may not exist).">"     
severity: "ERROR"     
sourceLocation: {
 file: "/base/data/home/apps/s~ProjectA/list-metrics:20200706t123743.427891295940019389/main.py"      
 functionName: "post"      
 line: "665"      
}
time: "2020-07-06T16:10:43.724399Z"

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

I have solved this in this way by adding the default service account of the app engine to the monitoring.viewer role for ProjectB and now I am able to get the metrics from ProjectB also