feat: Add the RDS performance and generic metrics option

[ZPascal]

Description

This PR extends the aliyuncms input plugin to support additional metric data sources beyond Cloud Monitor Service (CMS). Specifically, it adds support for collecting performance metrics directly from Alibaba Cloud's Relational Database Service (RDS) API.

Changes

• Added new metric_services configuration option to specify which metric services to query (defaults to ["cms"])
• Implemented RDS performance metrics collection via DescribeDBInstancePerformance API
• Refactored metric gathering to abstract data source selection between CMS and RDS
• Improved timestamp parsing to handle different formats from various APIs
• Enhanced code organization with extracted constants and helper functions
• Updated documentation with new configuration options

Key Features

• Multi-source metrics: Support for both CMS and RDS data sources
• Flexible configuration: Users can specify metric_services = ["cms", "rds"] to enable RDS metrics
• Backward compatible: Defaults to CMS-only behavior if metric_services is not configured
• RDS-specific handling: Parses compound metrics (e.g., CPUUsage&IOPS) and normalizes them into separate datapoints

Configuration Example

[[inputs.aliyuncms]] 
project = "acs_rds_dashboard" metric_services = ["cms", "rds"]
  [[inputs.aliyuncms.metrics]] 
  names = ["CPUUtilization", "MemoryUsage"] 
  dimensions = '{"instanceId": "rm-xxxxx"}'

Testing

• Added unit tests for RDS client mock
• Added test cases for single and compound metric parsing
• Added error handling tests for malformed responses
• Note: I will adapt the remaining unit tests once the general functionality is approved

Related Issues

Checklist

• Updated README.md with new configuration option
• Updated sample.conf with new configuration option
• Added initial unit tests for new functionality
• Complete unit test coverage (pending approval of approach)
• Code follows project style guidelines
• Backward compatible with existing configurations
• No AI generated code was used in this PR

Breaking Changes

None. The change is fully backward compatible with existing configurations.

Notes:

• The RDS metrics API has a different structure than CMS, requiring specialized parsing logic
• Compound metrics from RDS (separated by &) are automatically split into individual datapoints
• Unit tests are partially complete and will be fully updated once the general approach is confirmed

[telegraf-tiger]

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[ZPascal]

@srebhan Can you please check the PR?