Add --node flag to filter on a specific node

[tksm]

ref #242

This PR adds --node flag to filter on a specific node. This flag will be helpful when we debug pods on the specific node.

# Print a DaemonSet pod on the specific node
stern --node <NODE_NAME> daemonsets/<DS_NAME>

# Print all pods on the specific node
stern --node <NODE_NAME> --all-namespaces --no-follow --max-log-requests 1 .

I will implement a completion for this flag after this PR is merged.

Example

Let's say we have four nodes.

$ kubectl get nodes
NAME                 STATUS   ROLES           AGE   VERSION
kind-control-plane   Ready    control-plane   14m   v1.25.3
kind-worker          Ready    <none>          13m   v1.25.3
kind-worker2         Ready    <none>          13m   v1.25.3
kind-worker3         Ready    <none>          13m   v1.25.3

Without --node, stern shows all pods that the DaemonSet created.

$ stern -n kube-system --tail 1 --no-follow --only-log-lines daemonset/kindnet
kindnet-flqlg kindnet-cni I0305 01:34:00.843821       1 main.go:250] Node kind-worker3 has CIDR [10.244.1.0/24]
kindnet-5f4ms kindnet-cni I0305 01:33:59.442694       1 main.go:250] Node kind-worker3 has CIDR [10.244.1.0/24]
kindnet-bdgk4 kindnet-cni I0305 01:33:59.039534       1 main.go:227] handling current node
kindnet-hqn2k kindnet-cni I0305 01:33:59.268987       1 main.go:250] Node kind-worker3 has CIDR [10.244.1.0/24]

With --node kind-worker2, stern shows only the pod on the node kind-worker2.

$ stern --node kind-worker2 -n kube-system --tail 1 --no-follow --only-log-lines daemonset/kindnet
kindnet-hqn2k kindnet-cni I0305 01:36:19.251494       1 main.go:250] Node kind-worker3 has CIDR [10.244.1.0/24]

[rkmathi]

This looks very useful 😄

[tksm]

Thank you for the review!