* When HPAScaleToZero feature gate is enabled HPA supports scaling to zero pods based on object or external metrics. HPA remains active as long as at least one metric value available. ([#74526](https://github.com/kubernetes/kubernetes/pull/74526), [@DXist](https://github.com/DXist))
    * To downgrade the cluster to version that does not support scale-to-zero feature:

- Added an e2e test to verify that the cluster is not vulnerable to CVE-2021-29923 when using Services with IPs with leading zeros, note that this test is a necessary but not sufficient condition, all the components in the clusters that consume IPs addresses from the APIs MUST interpret them as decimal or discard them. ([#107552](https://github.com/kubernetes/kubernetes/pull/107552),...

- Added warnings to the Services API. Kubernetes now warns for Services in the case of:
  - IPv4 addresses with leading zeros
  - IPv6 address in non-canonical format (RFC 5952) ([#114505](https://github.com/kubernetes/kubernetes/pull/114505), [@aojea](https://github.com/aojea))

  the garbage collector starts to manage it.


#### Monitoring/Prometheus
* [action required] The WATCHLIST calls are now reported as WATCH verbs in prometheus for the apiserver_request_* series.  A new "scope" label is added to all apiserver_request_* values that is either 'cluster', 'resource', or 'namespace' depending on which level the query is performed at.([#52237](https://github.com/kubernetes/kubernetes/pull/52237))

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     * ensure that completed write operations performed by other threads are noticed. For most
     * purposes, the "count" field, tracking the number of elements, serves as that volatile
     * variable ensuring visibility. This is convenient because this field needs to be read in many
     * read operations anyway:
     *

* Splits Juju Charm layers into master/worker roles ([#40324](https://github.com/kubernetes/kubernetes/pull/40324), [@chuckbutler](https://github.com/chuckbutler))
    * Adds support for 1.5.x series of Kubernetes
    * Introduces a tactic for keeping templates in sync with upstream eliminating template drift
    * Adds CNI support to the Juju Charms
    * Adds durable storage support to the Juju Charms

  - The `export` query parameter for list API calls, deprecated since v1.14, will be removed in v1.18.
  - The `series.state` field in the events.k8s.io/v1beta1 Event API is deprecated and will be removed in v1.18 ([#75987](https://github.com/kubernetes/kubernetes/pull/75987), [@yastij](https://github.com/yastij))

- kubeadm

- Update the Event series starting count when emitting isomorphic events from 1 to 2. ([#119375](https://github.com/kubernetes/kubernetes/pull/119375), [@dgrisonnet](https://github.com/dgrisonnet)) [SIG API Machinery and Testing]

## Dependencies

- Ubuntu 18.04 (Bionic) series has been added to Juju charms ([#65644](https://github.com/kubernetes/kubernetes/pull/65644), [@tvansteenburgh](https://github.com/tvansteenburgh))