[ISSUE #_METRICS-01] Add Prometheus alert rules pack for RocketMQ monitoring

src/main/resources/prometheus/README.md

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+# RocketMQ Prometheus Alert Rules
+
+This directory contains production-ready [Prometheus](https://prometheus.io/) alerting rule
+definitions for Apache RocketMQ 5.x.
+
+## Files
+
+| File | Description |
+|---|---|
+| `rocketmq-alerts.yaml` | 22 alert rules covering broker availability, consumer lag, throughput anomalies, connection issues, resource saturation, transaction health, and message size. |
+
+## Metric Source
+
+All metric names and labels are sourced from the official OpenTelemetry-based exporter
+embedded in RocketMQ broker and proxy since version 5.x:
+
+- **Broker metrics**: `broker/src/main/java/org/apache/rocketmq/broker/metrics/BrokerMetricsConstant.java`
+- **Proxy metrics**: `proxy/src/main/java/org/apache/rocketmq/proxy/metrics/ProxyMetricsConstant.java`
+
+To enable the built-in Prometheus endpoint on the broker, set the following in `broker.conf`:
+
+```properties
+metricsExporterType=PROM
+```
+
+This exposes metrics at `http://<broker-host>:5557/metrics`.
+
+For the proxy, set in `rmq-proxy.json`:
+
+```json
+{
+  "metricsExporterType": "PROM"
+}
+```
+
+## Alert Categories
+
+| Category | # Rules | Key Metrics |
+|---|---|---|
+| Broker Availability | 2 | `rocketmq_broker_permission`, `rocketmq_processor_watermark` |
+| Proxy Availability | 2 | `rocketmq_proxy_up` |
+| Consumer Lag | 5 | `rocketmq_consumer_lag_messages`, `rocketmq_consumer_lag_latency`, `rocketmq_consumer_inflight_messages`, `rocketmq_consumer_queueing_latency` |
+| Throughput Anomalies | 3 | `rocketmq_messages_in_total`, `rocketmq_messages_out_total`, `rocketmq_send_to_dlq_messages_total` |
+| Connection Anomalies | 2 | `rocketmq_producer_connections`, `rocketmq_consumer_connections` |
+| Resource Saturation | 2 | `rocketmq_topic_number`, `rocketmq_consumer_group_number` |
+| Transaction Health | 2 | `rocketmq_half_messages`, `rocketmq_rollback_messages_total` |
+| Message Size | 1 | `rocketmq_message_size` |
+| Throughput Bandwidth | 2 | `rocketmq_throughput_in_total`, `rocketmq_throughput_out_total` |
+| Transaction Latency | 1 | `rocketmq_finish_message_latency` |
+
+## Loading Alert Rules
+
+### Option 1: Via `prometheus.yml`
+
+```yaml
+rule_files:
+  - /path/to/rocketmq-alerts.yaml
+```
+
+Restart or reload Prometheus:
+
+```bash
+curl -X POST http://localhost:9090/-/reload
+```
+
+### Option 2: Via Docker
+
+Mount the rules file and reference it in `prometheus.yml`:
+
+```yaml
+# docker-compose.yml
+services:
+  prometheus:
+    image: prom/prometheus:latest
+    volumes:
+      - ./prometheus.yml:/etc/prometheus/prometheus.yml
+      - ./rocketmq-alerts.yaml:/etc/prometheus/rocketmq-alerts.yaml
+    ports:
+      - "9090:9090"
+```
+
+### Option 3: Via Helm (kube-prometheus-stack / Prometheus Operator)
+
+kube-prometheus-stack loads alerting rules via `PrometheusRule` resources (not ConfigMaps or `prometheusSpec.ruleFiles`).
+
+    apiVersion: monitoring.coreos.com/v1
+    kind: PrometheusRule
+    metadata:
+      name: rocketmq-alerts
+      labels:
+        release: <your-helm-release-name>
+    spec:
+      groups:
+        # paste the `groups:` section from rocketmq-alerts.yaml here
+
+## Customizing Thresholds
+
+All thresholds are defined as generic defaults. Adjust them according to your
+specific workload and SLA requirements:
+
+| Alert | Default Threshold | Label for Customization |
+|---|---|---|
+| `RocketMQConsumerLagMessagesHigh` | > 10 000 | Filter by `topic` or `consumer_group` |
+| `RocketMQConsumerLagMessagesCritical` | > 100 000 | Filter by `topic` or `consumer_group` |
+| `RocketMQConsumerLagLatencyHigh` | > 10 min (600 000 ms) | Filter by `topic` or `consumer_group` |
+| `RocketMQDLQRateHigh` | > 10 msgs/s | Filter by `consumer_group` |
+| `RocketMQInboundBandwidthHigh` | > 100 MB/s | Filter by `node_id` |
+| `RocketMQTopicCountHigh` | > 10 000 | Per broker |
+
+Example — only alert on a specific topic:
+
+```yaml
+expr: rocketmq_consumer_lag_messages{topic="my-important-topic"} > 5000
+```
+
+## Severity Levels
+
+- **critical**: Immediate action required — broker/proxy down, severe backlog,
+  permission degraded.
+- **warning**: Investigate soon — performance degradation, connection loss,
+  accumulating resources.
+
+## Integration with Alertmanager
+
+Recommended `alertmanager.yml` route:
+
+```yaml
+route:
+  group_by: ['alertname', 'cluster', 'node_id']
+  group_wait: 30s
+  group_interval: 5m
+  repeat_interval: 4h
+  receiver: 'default'
+
+receivers:
+  - name: 'default'
+    # Configure your notification channel (Slack, PagerDuty, email, etc.)
+```