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-deployment.yaml

@@ -24,9 +24,6 @@ spec:
       containers:
         - name: opensearch
           image: opensearchproject/opensearch:3
-          # OpenSearch 3.x has no maintained prometheus-exporter plugin.
-          # Metrics come from /_cluster/stats JSON API (scraped by dashboard queries).
-          # TODO: add opensearch-exporter sidecar for native Prometheus metrics.
           ports:
             - name: http
               containerPort: 9200
@@ -58,6 +55,26 @@ spec:
           volumeMounts:
             - name: data
               mountPath: /usr/share/opensearch/data
+        # Prometheus metrics exporter — scrapes OpenSearch REST APIs and
+        # exposes them as elasticsearch_* metrics on :9114/metrics.
+        - name: exporter
+          image: quay.io/prometheuscommunity/elasticsearch-exporter:v1.7.0
+          args:
+            - --es.uri=http://localhost:9200
+            - --es.all
+            - --es.indices
+            - --es.shards
+            - --collector.clustersettings
+          ports:
+            - name: metrics
+              containerPort: 9114
+              protocol: TCP
+          resources:
+            requests:
+              cpu: 5m
+              memory: 32Mi
+            limits:
+              memory: 64Mi
       volumes:
         - name: data
           persistentVolumeClaim: