Empty external metrics list will fail the client-go CachedDiscoveryClient caching and make helm upgrade resending api call

[jacobycwang]

The custom-metrics-stackdriver-adapter returns an empty list in ListAllExternalMetrics (e.g., kubectl get --raw '/apis/external.metrics.k8s.io/v1beta1'). Although this allows the HPA to function, it causes side effects in other Kubernetes ecosystem tools, such as Helm.

k8s-stackdriver/custom-metrics-stackdriver-adapter/pkg/adapter/provider/provider.go

Lines 354 to 358 in d0e6643

	// ListAllExternalMetrics returns a list of available external metrics.
	// Not implemented (currently returns empty list).
	func (p *StackdriverProvider) ListAllExternalMetrics() []provider.ExternalMetricInfo {
	return []provider.ExternalMetricInfo{}
	}

We encountered an issue where helm upgrade sends over 400 API requests. This is because Helm relies on the client-go CachedDiscoveryClient for validation. CachedDiscoveryClient caches responses and saves them to a file if the response is not empty. When the custom-metrics-stackdriver-adapter returns an empty list for external.metrics.k8s.io/v1beta1, it is never cached, causing the CachedDiscoveryClient to continuously refresh all API resources (not just external.metrics.k8s.io/v1beta1). This results in a high number of API requests being sent.
https://github.com/kubernetes/client-go/blob/c7396197f39ed43fb0369c054360fcb989d65c88/discovery/cached/disk/cached_discovery.go#L82C35-L82C65
https://github.com/kubernetes/client-go/blob/c7396197f39ed43fb0369c054360fcb989d65c88/discovery/cached/memory/memcache.go#L104

[CatherineF-dev]

From grafana/tanka#1076, it might be related to #690 which bumped client-go version to fix vulnerabilities.

[CatherineF-dev]

Could you try this image: gcr.io/gke-release/custom-metrics-stackdriver-adapter:v0.14.2-gke.0 ?

[raywainman]

Helm recently upgraded from client-go v0.29 to v0.30 in helm/helm#13021.

[CatherineF-dev]

Adding a fake metric here #743

[CatherineF-dev]

Hi, we will release a new image version soon.
Main branch is still using the old image version.