Add cluster attribute to nais_api_watcher_resources metric

[Starefossen]

Problem

nais_api_watcher_resources only has type and action attributes. We cannot filter or group resource counts by cluster in dashboards.

Suggestion

The cluster string is already available in add(), remove(), and update() in internal/kubernetes/watcher/watcher.go — just pass it as an attribute:

w.resourceCounter.Add(context.TODO(), 1, metric.WithAttributes(
    attribute.String("type", w.watchedType),
    attribute.String("action", "add"),
    attribute.String("cluster", cluster), // <-- add this
))

Same change in remove() and update().

Why

This enables tenant dashboards to show per-cluster resource adoption (apps, topics, CloudSQL, buckets, valkey, opensearch) without needing a separate query per cluster.