monitoringpb: types for cloudRun, gkeNamespace, gkeWorkload and gkeService are missing

**Is your feature request related to a problem? Please describe.**
I expect to use the monitoring client to create services of type `gkeWorkload`.

**Describe the solution you'd like**
Add the missing types to the library.

**Describe alternatives you've considered**
Should I implement the missing types by myself?

**Additional context**
The type is listed in the REST/gRPC Reference, so I kind of expect this to be available here as well: https://cloud.google.com/monitoring/api/ref_v3/rest/v3/services#gkeworkload or https://cloud.google.com/monitoring/api/ref_v3/rpc/google.monitoring.v3#google.monitoring.v3.Service.