Merge tag '0.4.0' into debian/yoga

oslo.metrics 0.4.0 release

meta:version: 0.4.0
meta:diff-start: -
meta:series: yoga
meta:release-type: release
meta:pypi: yes
meta:first: yes
meta:release:Author: Hervé Beraud <hberaud@redhat.com>
meta:release:Commit: Hervé Beraud <hberaud@redhat.com>
meta:release:Change-Id: Ic6d479ec8720cdf5fcdb958dde3b66669bf7a2e0
meta:release:Code-Review+2: Elod Illes <elod.illes@est.tech>
meta:release:Code-Review+2: Thierry Carrez <thierry@openstack.org>
meta:release:Workflow+1: Thierry Carrez <thierry@openstack.org>