[receiver/rabbitmq] Enhancement/rabbitmq Added node level metrics

[VenuEmmadi]

Description

This PR enhances the RabbitMQ receiver by adding support for node-level metrics collection. It introduces the enable_node_metrics flag in the configuration, allowing users to enable or disable the collection of node-specific metrics such as:

• rabbitmq.node.disk_free: Free disk space on the node.
• rabbitmq.node.fd_used: The number of file descriptors used on the node.
• rabbitmq.node.mem_limit: The memory limit on the node.
• rabbitmq.node.mem_used: The memory used on the node.

This enhancement improves the flexibility and granularity of the metrics collection for RabbitMQ monitoring.

Link to tracking issue

Fixes #36925

Testing

• Added unit tests to validate the enable_node_metrics flag functionality.
• Verified that metrics are collected correctly when the flag is enabled and omitted when disabled.
• Ran integration tests to ensure the receiver operates without issues in various configurations.

Documentation

• Updated the README.md with details about the new enable_node_metrics flag.
• Added examples in config.yaml to demonstrate how to configure the receiver with the new flag.
• Included explanations for node-specific metrics in metadata.yaml.