fix(ansible): name the daemon's log stream decdn-node

[thiras]

Summary

• loki.relabel "journal_identity" adds a replace rule: if unit matches decdn-node\.service, it sets service_name to grafana_alloy_service_name. Other units don't match, so they keep their current labels. job stays on the value the identity rules set (by default integrations/node_exporter), so the Linux Server log dashboards still match. The rule isn't rendered when grafana_alloy_service_name is empty.

• The README now lists the three places Alloy sets service_name:

  • the node's /metrics series;
  • everything received on the local OTLP endpoint, as service.name, overwriting the sender's value;
  • the daemon's journald stream.

  It also covers two caveats:

  • systemd's own start/stop/crash messages about the daemon are logged under unit="init.scope", so {service_name="decdn-node"} won't show a crash loop;
  • Grafana Cloud's Loki may fill in a service_name for streams that don't have one.

• Rewrote the "EVERY series/span/log line" wording in defaults, README and preflight. It contradicted the new rule and was already slightly wrong.

Tests

• validate.sh now checks, on defaults.alloy, the rule's source_labels, regex, target_label and replacement. These checks depend on column alignment, and a comment says why.
• The minimal fixture in render.yml now sets grafana_alloy_service_name: "". On that render, validate.sh checks that the metric rule, the journald rule and the OTTL service.name statement are all gone, and that the journald identity relabeller is still there.

Test plan

• make lint-alloy passes: the real pinned Alloy v1.19.2 accepts all 9 renders, and every check passes.
• Removing the template rule makes the defaults checks fail. Removing the Jinja guard makes the blank-service_name check fail.
• make lint (pre-commit) and make lint-ansible (production profile) pass.

🤖 Generated with Claude Code

[Copilot]

Copilot review overview

🟡 Changes recommended

Add coverage for logs-enabled renders with an empty service name.

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Review effort: Lite
Findings: 1

Open (1)

• Add coverage for logs-enabled empty service name · New

What changed in this PR

Updates Grafana Alloy journald relabeling so only decdn-node.service receives service_name, with documentation and validation updates.

Changes:

• Adds daemon-specific Loki relabeling.
• Updates README and defaults documentation.
• Extends rendered-config validation.

File	Summary
ansible/​tests/​alloy-config/​validate.sh	Validates the new relabel rule.
ansible/​roles/​grafana_alloy/​templates/​config.alloy.j2	Applies labeling only to daemon logs.
ansible/​roles/​grafana_alloy/​README.md	Documents resulting log labels.
ansible/​roles/​grafana_alloy/​defaults/​main.yml	Clarifies label behavior.

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