k8s: Add capability flag for watching metav1.POM

The ability to watch metav1.PartialObjectMetadata (or POM) and
metav1.Table was introduced in K8s 1.15. [1]

This is relevant because our CRD controller attempts to fetch CRDs in
the cluster efficiently by requesting the CRD in a POM object. The CRD
controller does this in order to avoid requesting the full object, which
may contain a large validation schema and other irrelevant fields. This
is important because in large-scale environments, all agents will
request all the CRDs at once which will put unnecessary load on the
apiserver.

However, we cannot perform this request at all on versions of K8s
1.14 and below. Therefore, we must fullback to requesting the full CRD
object. Hence, this commit allows us to check whether the apiserver
supports this action, so that we can efficiently request CRDs on
versions that do support it.

[1]: KEP:
https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/20190322-server-side-get-to-ga.md#goals

[1]: PR: kubernetes/kubernetes#71548

Signed-off-by: Chris Tarazi <chris@isovalent.com>

pkg/k8s/version/version.go

@@ -63,6 +63,19 @@ type ServerCapabilities struct {
 	// This capability was introduced in K8s version 1.16, prior to which
 	// apiextensions/v1beta1 CRDs were used exclusively.
 	APIExtensionsV1CRD bool
+
+	// WatchPartialObjectMetadata is set to true when the K8s server supports a
+	// watch operation on the metav1.PartialObjectMetadata (and metav1.Table)
+	// resource.
+	//
+	// This capability was introduced in K8s version 1.15, prior to which
+	// watches cannot be performed on the aforementioned resources.
+	//
+	// Source:
+	//   - KEP:
+	//   https://github.com/kubernetes/enhancements/blob/master/keps/sig-api-machinery/20190322-server-side-get-to-ga.md#goals
+	//   - PR: https://github.com/kubernetes/kubernetes/pull/71548
+	WatchPartialObjectMetadata bool
 }
 
 type cachedVersion struct {
@@ -94,6 +107,10 @@ var (
 	// v1 CRDs was introduced in K8s version 1.16.
 	isGEThanAPIExtensionsV1CRD = versioncheck.MustCompile(">=1.16.0")
 
+	// Constraint to check support for watching metav1.PartialObjectMetadata
+	// and metav1.Table types. Support was introduced in K8s 1.15.
+	isGEThanWatchPartialObjectMeta = versioncheck.MustCompile(">=1.15.0")
+
 	// isGEThanMinimalVersionConstraint is the minimal version required to run
 	// Cilium
 	isGEThanMinimalVersionConstraint = versioncheck.MustCompile(">=" + MinimalVersionConstraint)
@@ -124,6 +141,7 @@ func updateVersion(version go_version.Version) {
 	cached.capabilities.Patch = option.Config.K8sForceJSONPatch || isGEThanPatchConstraint(version)
 	cached.capabilities.MinimalVersionMet = isGEThanMinimalVersionConstraint(version)
 	cached.capabilities.APIExtensionsV1CRD = isGEThanAPIExtensionsV1CRD(version)
+	cached.capabilities.WatchPartialObjectMetadata = isGEThanWatchPartialObjectMeta(version)
 }
 
 func updateServerGroupsAndResources(apiResourceLists []*metav1.APIResourceList) {