feat(data): OpenSearch prometheus-exporter sidecar

elasticsearch-exporter v1.7.0 runs as a sidecar, scrapes localhost:9200,
exposes elasticsearch_* metrics on :9114. ServiceMonitor re-enabled.
Alert rules updated to use elasticsearch_* metric names.
Flags: --es.all --es.indices --es.shards --collector.clustersettings

base/data/opensearch-servicemonitor.yaml

@@ -11,6 +11,5 @@ spec:
     matchLabels:
       app: opensearch
   endpoints:
-    - port: http
+    - port: metrics
       interval: 30s
-      path: /_prometheus/metrics