helm: add service_name relabel rule to alloy scrape config

[SLoeuillet]

What

Adds a relabel rule to the Helm chart's default Alloy scrape config that maps the profiles.grafana.com/service_name pod annotation to the service_name label:

rule {
    action        = "replace"
    source_labels = ["__meta_kubernetes_pod_annotation_profiles_grafana_com_service_name"]
    target_label  = "service_name"
}

It sits alongside the existing service_repository / service_git_ref / service_root_path rules in templates/configmap-alloy.yaml.

Why

The chart already lets users set service_repository, service_git_ref and service_root_path via profiles.grafana.com/* annotations, but not service_name — the single most important label, used by the App Selector to identify an application (#2051).

Without it, Alloy falls back to inferring service_name from target metadata (namespace + app.kubernetes.io/*). When that inference is generic — e.g. a Helm component literally named default — pods surface under a useless/colliding service_name (<namespace>/default) with no annotation-based way to override it, unlike every other service_* label. This rule closes that gap so an operator can simply set profiles.grafana.com/service_name: my-app.

Behaviour / compatibility

• No-op unless a pod sets the annotation. An explicit service_name label takes precedence over Alloy's inference, so existing deployments are completely unaffected until they opt in.
• Consistent with the existing service_* annotation rules — same block, same placement.

Changes

• templates/configmap-alloy.yaml — the new rule.
• rendered/*.yaml — regenerated (the rule is in the static config block, so it appears identically in all six fixtures).
• Chart.yaml / README.md — chart version 2.0.3 → 2.0.4 (appVersion unchanged).

Closes #2051

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