API the server is currently unable to handle the request

Hi

i have GKE v1.18.12-gke.1206 (happened also on v1.17.14-gke.400)
its installed by default with metric servier v0.3.6 and custom-metrics-stackdriver-adapter v0.10.2
i recently see a lot of issues where HPA are failing to fetch custom metrics and i see the following errors:

```
unable to fetch metrics from external metrics API: the server is currently unable to handle the request (get custom.googleapis.com|rabbitmq|queue_messages.external.metrics.k8s.io)
```

also the following
```
unable to fetch metrics from external metrics API: the server was unable to return a response in the time allotted, but may still be processing the request
```

is this a know issue? the custom-metrics-stackdriver-adapter pod includes many Errors such as `apiserver received an error that is not an metav1.Status: &errors.errorString{s:"http2: stream closed"}` (there is another thread on that) so its hard to tell if its related or not.

any idea?
thanks