added notes on available RE metrics

Author: maggiehays

docs/managed-datahub/release-notes/v_0_3_9.md

@@ -49,6 +49,13 @@ Helm Chart
 
 - Platform
 
+  - Remote Executor: Introduced new Prometheus Metrics for customer-side monitoring:
+    - Ingestion: `datahub_executor_worker_ingestion_requests` and `datahub_executor_worker_ingestion_errors` to monitor ingestion jobs received from queue
+    - Memory: `datahub_executor_memory_*` to monitor usage and limits by process group/host
+    - CPU: `datahub_executor_cpu_*` to monitor usage and limits by process group/host
+    - Disk: `datahub_executor_disk_*` to monitor total bytes and bytes used
+    - Network: `datahub_executor_net_*` to monitor bytes and packets sent/received, etc. 
+
 - Ingestion 
   - MLflow: Support for experiments & runs ingestion, enhanced property ingestion for models/model versions with native versioning support
     - Dataset ↔ Run lineage is coming soon.