[metricbeat] Add priorityClassName config

[gadiener]

This PR add a priorityClassName config to metricbeat helm chart.

• Chart version not bumped (the versions are all bumped and released at the same time)
• README.md updated with any new values or changes
• Updated template tests in ${CHART}/tests/*.py
• Updated integration tests in ${CHART}/examples/*/test/goss.yaml

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

Author of the following commits did not sign a Contributor Agreement:
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Please, read and sign the above mentioned agreement if you want to contribute to this project

[elasticcla]

Hi @gadiener, we have found your signature in our records, but it seems like you have signed with a different e-mail than the one used in your Git commit. Can you please add both of these e-mails into your Github profile (they can be hidden), so we can match your e-mails to your Github profile?

[pbecotte]

nice!

[jmlrt]

Thank you for this PR.
If we add priorityClassName for Metricbeat daemonset, we should also add it for Metricbeat deployment.

[jmlrt]

jenkins test this please

[jmlrt]

jenkins test this please

[jmlrt]

jenkins test this please

[gadiener]

Hi @jmlrt, I added priorityClassName to the Metricbeat deployment. I've also updated the unit test.

[jmlrt]

LGTM