focused on stability and performance improvements in the 1.13 release.  A new alpha feature is introduced to improve the mechanism that nodes heartbeat back to the control plane.  The `NodeLease` feature results in the node using a `Lease` resource in the `kube-node-lease` namespace that is renewed periodically.  The `NodeStatus` that was used previously to heartbeat back to the control plane is only updated when it changes.  This reduces load on the control plane for large clusters. The Kubelet plugin...

- kubelet: fix fail to close kubelet->API connections on heartbeat failure when bootstrapping or client certificate rotation is disabled ([#78016](https://github.com/kubernetes/kubernetes/pull/78016), [@gaorong](https://github.com/gaorong))

}

// EventSeries contain information on series of events, i.e. thing that was/is happening
// continuously for some time.
message EventSeries {
  // Number of occurrences in this series up to the last heartbeat time
  optional int32 count = 1;

  // Time of the last occurrence observed
  optional k8s.io.apimachinery.pkg.apis.meta.v1.MicroTime lastObservedTime = 2;
}

- The kubelet no longer forcefully closes active connections on heartbeat failures, using the HTTP2 health check mechanism to detect broken connections. Users can force the previous behavior of the kubelet by setting the environment variable DISABLE_HTTP2. ([#108107](https://github.com/kubernetes/kubernetes/pull/108107),...

* fix azure retry issue when return 2XX with error ([#78298](https://github.com/kubernetes/kubernetes/pull/78298), [@andyzhangx](https://github.com/andyzhangx))
* kubelet: fix fail to close kubelet->API connections on heartbeat failure when bootstrapping or client certificate rotation is disabled ([#78016](https://github.com/kubernetes/kubernetes/pull/78016), [@gaorong](https://github.com/gaorong))

* Added a way to pass extra arguments to etcd. The these extra arguments can be used to adjust runtime configuration like heartbeat interval etc. ([#63961](https://github.com/kubernetes/kubernetes/pull/63961), [@mborsz](https://github.com/mborsz))