feat(TargetAllocator): allow configuration of PrometheusCR namespaceSelectors

[dexter0195]

Description:
Adding the possibility to set values for PrometheusCR:

• probeNamespaceSelector
• serviceMonitorNamespaceSelector
• scrapeConfigNamespaceSelector
• probeNamespaceSelector

Link to tracking Issue(s): #3086

• Resolves: Unable to run the targetAllocator in namespace mode #3086

Testing: Added tests for configmap generation

Documentation: Updated README.md

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: dexter0195 / name: Alessio Trivisonno (b07ad6b, db97808, 7d1f53f)

[jaronoff97]

Change looks good overall, just one thought on how to prevent it from breaking. cc @swiatekm

[jaronoff97]

suggestion: we can remove the excess spacing here, it's superfluous IMO.

[jaronoff97]

I wonder if we should also do some defaulting here to prevent an upgrade from breaking existing installations? As the doc string states, a null selector matches nothing, upgrading with this on will mean that existing installations (who do not set this currently) would not have it, resulting in a null selector, aka 💥

[dexter0195]

Great point 👍 , should we implement that for the PodMonitorNamespaceSelector or for all the selectors (including the already existing ones) ? What do you recommend ?

[swiatekm]

We should set the default to the empty selector for all selectors you're adding here. The end result will be that resource selectors target nothing by default, while namespace selectors target everything, which is also what prometheus-operator does.

I'm not sure if you can set these defaults via a kubebuilder marker. If not, then you'll have to do it in the webhook.

[dexter0195]

Hey @swiatekm , thank you for the heads up for the kubebuilder 😃 I indeed found a way to set a default and I added it to the new fields like so

// +kubebuilder:default:={}

which resulted in a generated crd like this:

...
                      podMonitorNamespaceSelector:
                        default: {}
                        properties:
                          matchExpressions:
...

Let me know if that looks good for you 🙏