CloudWatch: Adding RDS Performance Insights and Enhanced Monitoring Metrics to AWS/RDS Namespace

<!-- Please only use this template for submitting feature requests --> **Why is this needed:** Monitoring RDS with Database Insights (including Performance Insights and Enhanced Monitoring Metrics) requires access to multiple metrics. Manually typing all metric names is inefficient and error-prone. **What needs to be added:** A dropdown menu that displays all Database Insights metrics under the AWS/RDS namespace. **Steps to reproduce:** 1. Enable Database Insights in RDS 2. Create a new CloudWatch datasource 3. Select AWS/RDS namespace 4. Note that Database Insights metrics are missing from the dropdown # As shown in the screenshot below, only default metrics are visible. <img width="2974" height="1572" alt="Image" src="https://github.com/user-attachments/assets/c85b80d3-d8bc-475f-ac2d-f0c30ae95c3b" /> # According to the AWS Public documentation [[1]](https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_PerfInsights_Counters.html), the metric 'Innodb_rows_deleted' (InnoDBRowsDeleted) is missing from the current Grafana Dashboard.