fix: add StoreBuilder to mimir.alerts.kubernetes eventProcessor

[synthe102]

PR Description

Which issue(s) this PR fixes

Fixes #4975

Notes to the Reviewer

Add an assets.StoreBuilder to the eventProcessor so it can be passed to provisionAlertmanagerConfiguration that needs to fetch k8s secrets and configmaps in order to populate Alertmanager configs.

PR Checklist

[CLAassistant]

All committers have signed the CLA.

[ptodev]

@synthe102 thank you! I added some integration and unit tests to your branch.

[synthe102]

Thanks @ptodev !

[ptodev]

@synthe102 I added an entry to the changelog and tagged you, I hope you don't mind.

[ptodev]

@synthe102 Do you mind rebasing your branch please?

[artemlive]

I've tested this, and there is another logical error. It's not directly related to the issue, but maybe it makes sense to fix it here.
When the config of the Alertmanager got generated, since we don't use the Alertmanager CRD, we built it with the defaults:

alloy/internal/component/mimir/alerts/kubernetes/events.go

Lines 178 to 180 in 8c0090c

	// TODO: Add an option to get an Alertmanager CRD through k8s informers.
	cfgBuilder = alertmanager.NewConfigBuilder(slog.New(logging.NewSlogGoKitHandler(c.logger)), *version, store, &monitoringv1.Alertmanager{})
	)

And it breaks the config, since it implicitly adds the

    matchers:
    - namespace="mimir"

to the main route:

...
 routes:
  - receiver: mimir/alertmanager-config/sre-alerts
    group_by:
    - alertname
    - cluster
    matchers:
    - namespace="mimir" #<---- the issue, since we care only about the alerts from the mimir ns
    continue: true
        routes:
          - receiver: mimir/alertmanager-config/other-alerts
            matchers:
            - cluster="staging"
            - project="project1"
            continue: false
            repeat_interval: 12h
...

Because we don't use the Alertmanager CRD, where we can set the alertmanagerConfigMatcherStrategy.
If you think it's unrelated, I can create a new issue.
Thanks.

[ptodev]

Hi @artemlive, do you mind opening another issue for this please?

[artemlive]

@ptodev sure thing
I've created the new issue.