Update single Cluster label via API

[agrrh]

Summary

It would be great to make it possible to add, set or remove cluster labels with an atomic API request.

Motivation

I operate number of cluster with personal operator, which calls ArgoCD API.

It sometimes need to add and remove cluster labels (changing where to roll or wipe ApplicationSet out).

There's PATCH method for Apps, but for Clusters, there's only PUT.

So, now my only way is to:

1. GET cluster, extract {labels}
2. Modify labels
3. PUT it back, sending modified {labels} dict using ?updatedFields=labels

Why it's not great:

It overwrites whole labels dict.
There's small but still a real chance that someone would edit labels between steps 1 and 3.
Thus, I would overwrite someone's data.

Proposal

I can think of three approaches here.

1. Add separate endpoints:

PUT    /api/v1/clusters/{id.value}/labels

[
  {"key": "example.org/foo", "value": "42"},
  {"key": "example.org/bar", "value": "baz"}
]

DELETE /api/v1/clusters/{id.value}/labels

2. Allow to run PATCH as in Application's PATCH, with JSON or merge patches, e.g.:

PATCH /api/v1/clusters/{id.value}

{"metadata": { "labels": { "example.org/foo": "42" } }}

3. Make it possible to run PUT on /clusters in more precise way, e.g.:

PUT /api/v1/clusters/{id.value}?updatedFields=labels%5B%22example.org%2Ffoo%22%5D%2Clabels%5B%22example.org%2Fbar%22%5D

# This ugly value means:
#   labels["example.org/foo"],labels["example.org/bar"]