Add modern labels to pods controlled by deployments etc.

[samyuh]

Some labels are missing in the pods, such as:

    app.kubernetes.io/name: "jupyterhub"
    app.kubernetes.io/instance: "release-name"
    app.kubernetes.io/managed-by: Helm

The issue occurred because 'jupyterhub.matchLabels' was imported instead of 'jupyterhub.labels'.

Closes #3595

[consideRatio]

Oh there were plenty of resources bugged like this - oops

[samyuh]

Yup, I reviewed all of them :)

[consideRatio]

Hmmm okay so this is the catch and why it wasnt done. We don't want to always restart pods just because we upgrade, and some labels added now will trigger a restart now no matter what - those with chart versions in them.

I think chart and heritage shouldn't be added, either old/new version of those labels

[samyuh]

Yeah it makes sense. I will look into a way to have this working without having chart and heritage

[consideRatio]

I'll refer to legacy and modern "match labels" below, with it I mean:

• legacy match labels: component, app, release
• modern match labels: app.kubernetes.io/component, app.kubernetes.io/name, app.kubernetes.io/instance

Note that I don't think pods should have the old heritage label or the modern app.kubernetes.io/managed-by label, because they are actually managed by the Deployment etc, which in turn is managed by Helm. Currently they only have the legacy match labels not including heritage.

I think currently we have a need for:

• specifying legacy match labels on spec.selector.matchLabels as we already do, which is done by including the jupyterhub.matchLabels helper.
• specifying legacy and new match labels on spec.template.metadata.labels, where we currently just provide the legacy labels, but want to provide both legacy and modern.

I think in the future, we'll have a need for specifying just the new match labels on spec.selector.matchLabels.

---

With this in mind, I think practically, we should have one new helper function named for example like jupyterhub.modernMatchLabels which is added to be included next to jupyterhub.matchLabels in spec.template.metadata.labels, and in the future, we'll replace use of jupyterhub.matchLabels in spec.selector.matchLabels with jupyterhub.modernMatchLabels.

[consideRatio]

@samyuh I hope I didn't cause trouble pushing a commit without a heads up - I was worried I wouldn't find time to review future work in the near future and wanted to get this resolved.

[samyuh]

No worries! I'm a bit tied up this weekend and haven't had a chance to work on this yet. I really appreciate your help! Thanks again for taking the time to fix this!