Add rule_files to prometheus.yml, remove empty node-exporter.rules (#46)

prometheus.yml was missing the rule_files section, so alerting rules
deployed to /usr/local/etc/prometheus/rules/ were never loaded.

- Add rule_files glob so Prometheus evaluates the ZFS pool rules
- Document that alerting notifications go through Grafana, not
  Alertmanager — no alerting: section needed
- Remove node-exporter.rules (all rules were commented out)

Resolves PESO-103

ansible/services/prometheus/prometheus.yml

@@ -8,6 +8,13 @@ global:
   scrape_interval: 15s
   evaluation_interval: 15s
 
+# Alerting notifications are handled by Grafana (unified alerting with
+# PagerDuty + email contact points), not Alertmanager.  No alerting:
+# section is needed here.  Prometheus still evaluates these rule_files
+# so the ALERTS / ALERTS_FOR_STATE metrics are available for queries.
+rule_files:
+  - /usr/local/etc/prometheus/rules/*.rules
+
 scrape_configs:
   - job_name: "prometheus"
     static_configs:

ansible/services/prometheus/rules/node-exporter.rules

@@ -1,9 +0,0 @@
-groups:
-- name: alerts
-  rules:
-#   - alert: ServerRunningBtrfs
-#     expr: node_btrfs_info > 0
-#     for: 5m
-#     annotations:
-#       summary: "Server {{ $labels.job }} is running btrfs"
-#       description: "{{ $labels.job }} has uuid of '{{ $labels.uuid }}'"