Changes since v0.8.4:

All official images for releases after v0.8.4 are available in [gcr.io](http://k8s.gcr.io/prometheus-adapter/prometheus-adapter). The project also maintains a [staging registry](https://console.cloud.google.com/gcr/images/k8s-staging-prometheus-adapter/GLOBAL/) where images for each commit from the master branch are published. You can use this registry if you need to test a version from a specific commit, or if you need to deploy a patch while waiting for a new release, but do note that the stability of these images is not guaranteed.

Images for versions v0.8.4 and prior are only available in unofficial registries:
- https://quay.io/repository/coreos/k8s-prometheus-adapter-amd64
- https://hub.docker.com/r/directxman12/k8s-prometheus-adapter/

Changes:
* Publish official images of prometheus-adapter to k8s.gcr.io and stop publishing to unofficial registries (@dgrisonnet)
* Move module to sigs.k8s.io/prometheus-adapter (@dgrisonnet)

Improvements:
* Allow external metrics to be defined as namespaced: false (@carsonoid)
* Support setting headers on requests to Prometheus (@discordianfish)
* Propagate metric providers context (@dgrisonnet)
* Update Kubernetes dependencies to v1.22 and golang version to v1.16 (@dgrisonnet)

Bug fixes:
* Prevent panics when prometheus-adapter can't get PodMetrics/NodeMetrics (@dgrisonnet)
* Filter non-running pods (@dgrisonnet)
* Prevent prometheus-adapter from returning negative resource metrics (@dgrisonnet)
* Fix incorrect type used for openapi spec (@arajkumar)
* Fix openapi spec for custom and external metrics types (@arajkumar)

Documentation:
* Update deploy/README.md to fix links (@andrewpollack)
* Update gcr promotion guidelines (@dgrisonnet)
* Default images to the official k8s.gcr.io and gcr.io registries (@dgrisonnet)
* Fix external rule tag in documentation (@ashishranjan1457)
* Document image registries (@fpetkovski)
* Update documentation to include metrics-max-age (@leoskyrocker)

Clean-ups:
* Remove vendor and localvendor directories (@dgrisonnet)
* Consolidate docker build (@dgrisonnet)
* Improve container push rules (@dgrisonnet)
* Move image publication to prow.k8s.io and remove Travis CI (@dgrisonnet)