I was struggling as well to get the actual counts. In the current UI , there might be some issue with selecting resources (I was not able to do so). Hence I used PromQL language.
Go to the metric explorer in monitoring.
select promQL on right hand side.
Enter this code:
sum by (response_code_class)(run_googleapis_com:request_count{monitored_resource="cloud_run_revision"})
Give the dashboard some time to refresh and collect the data , Later you can see the request cumulatively adding up based on you selected filter on time.