Metrics Server: use vendored OpenAPI definitions (kedacore#3928)

* Metrics Server: use vendored OpenAPI definitions

custom-metrics-apiserver serves OpenAPI spec by default since
version [v1.25.0] (cf [PR 110]).

[v1.25.0]: https://github.com/kubernetes-sigs/custom-metrics-apiserver/releases/tag/v1.25.0
[PR 110]: kubernetes-sigs/custom-metrics-apiserver#110

In Keda Metrics Server, remove generation of
`adapter/generated/openapi/zz_generated.openapi.go` and use
OpenAPI definitions from custom-metrics-apiserver instead.

Signed-off-by: Olivier Lemasle <olivier.lemasle@apalia.net>

* Update CHANGELOG.md

Co-authored-by: Zbynek Roubalik <zroubalik@gmail.com>
Signed-off-by: Olivier Lemasle <o.lemasle@gmail.com>

Signed-off-by: Olivier Lemasle <olivier.lemasle@apalia.net>
Signed-off-by: Olivier Lemasle <o.lemasle@gmail.com>
Co-authored-by: Zbynek Roubalik <zroubalik@gmail.com>

CHANGELOG.md

@@ -92,6 +92,7 @@ To learn more about active deprecations, we recommend checking [GitHub Discussio
 - **General**: Bump Golang to 1.18.6 ([#3205](https://github.com/kedacore/keda/issues/3205))
 - **General**: Bump `github.com/Azure/azure-event-hubs-go/v3` ([#2986](https://github.com/kedacore/keda/issues/2986))
 - **General**: Migrate from `azure-service-bus-go` to `azservicebus` ([#3394](https://github.com/kedacore/keda/issues/3394))
+- **General**: Metrics Server: use OpenAPI definitions served by custom-metrics-apiserver ([#3929](https://github.com/kedacore/keda/issues/3929))
 - **Azure EventHub**: Add e2e tests ([#2792](https://github.com/kedacore/keda/issues/2792))
 
 ## v2.8.1

Makefile

@@ -123,16 +123,6 @@ manifests: controller-gen ## Generate ClusterRole and CustomResourceDefinition o
 generate: controller-gen mockgen-gen proto-gen ## Generate code containing DeepCopy, DeepCopyInto, DeepCopyObject method implementations (API), mocks and proto.
 	$(CONTROLLER_GEN) object:headerFile="hack/boilerplate.go.txt" paths="./..."
 
-adapter/generated/openapi/zz_generated.openapi.go: go.mod go.sum ## Generate OpenAPI for KEDA Metrics Adapter.
-	go run vendor/k8s.io/kube-openapi/cmd/openapi-gen/openapi-gen.go --logtostderr \
-	    -i k8s.io/metrics/pkg/apis/custom_metrics,k8s.io/metrics/pkg/apis/custom_metrics/v1beta1,k8s.io/metrics/pkg/apis/custom_metrics/v1beta2,k8s.io/metrics/pkg/apis/external_metrics,k8s.io/metrics/pkg/apis/external_metrics/v1beta1,k8s.io/metrics/pkg/apis/metrics,k8s.io/metrics/pkg/apis/metrics/v1beta1,k8s.io/apimachinery/pkg/apis/meta/v1,k8s.io/apimachinery/pkg/api/resource,k8s.io/apimachinery/pkg/version,k8s.io/api/core/v1 \
-	    --build-tag autogenerated \
-	    -h ./hack/boilerplate.go.txt \
-	    -p ./adapter/generated/openapi \
-	    -O zz_generated.openapi \
-	    -o ./ \
-	    -r /dev/null
-
 fmt: ## Run go fmt against code.
 	go fmt ./...
 
@@ -177,7 +167,7 @@ build: generate fmt vet manager adapter ## Build Operator (manager) and Metrics
 manager: generate
 	${GO_BUILD_VARS} go build -ldflags $(GO_LDFLAGS) -mod=vendor -o bin/keda main.go
 
-adapter: generate adapter/generated/openapi/zz_generated.openapi.go
+adapter: generate
 	${GO_BUILD_VARS} go build -ldflags $(GO_LDFLAGS) -mod=vendor -o bin/keda-adapter adapter/main.go
 
 run: manifests generate ## Run a controller from your host.