support for restricting k8s discovery with exclusions

[gberche-orange]

Expected behavior

As a coroot user,
In order to support large mutualized k8s clusters, with many nodes and pods
I need to be able to add some exclusions to the k8s discovery (by node selector, or pod selectors)

Currently, As a 1st step I'm considering not deploying coroot agent on some nodes through
https://github.com/coroot/helm-charts/blob/e9067ce3d486d17f73391b093b8b8c1cd2588944/charts/coroot/templates/cluster-agent.yaml#L109-L119

Current behavior

Coroot agent seems to watch for all pods without possible exclusion

coroot-cluster-agent/discovery/k8s.go

Lines 81 to 115 in c22497f

	func (k8s *K8S) start() {
	watcher := cache.NewListWatchFromClient(k8s.client.CoreV1().RESTClient(), "pods", metav1.NamespaceAll, fields.Everything())
	_, informer := cache.NewInformer(watcher, &corev1.Pod{}, 0, cache.ResourceEventHandlerFuncs{
	AddFunc: func(obj interface{}) {
	pod := podFromObj(obj)
	if !pod.Running() {
	return
	}
	k8s.lock.Lock()
	defer k8s.lock.Unlock()
	k8s.pods[pod.Id] = pod
	k8s.sendEvent(PodEvent{Type: PodEventTypeAdd, Curr: pod})
	},
	UpdateFunc: func(oldObj, newObj interface{}) {
	pod := podFromObj(newObj)
	prev := podFromObj(oldObj)
	if !pod.Running() || pod.Equal(prev) {
	return
	}
	k8s.lock.Lock()
	defer k8s.lock.Unlock()
	k8s.pods[pod.Id] = pod
	k8s.sendEvent(PodEvent{Type: PodEventTypeChange, Curr: pod, Prev: prev})
	},
	DeleteFunc: func(obj interface{}) {
	pod := podFromObj(obj)
	k8s.lock.Lock()
	defer k8s.lock.Unlock()
	delete(k8s.pods, pod.Id)
	k8s.sendEvent(PodEvent{Type: PodEventTypeDelete, Curr: pod})
	},
	})
	go informer.Run(k8s.stopCh)
	}