[receiver/rabbitmq] Enhance rabbitmqreceiver to collect full set of node-level metrics from /api/nodes (open-telemetry#38998)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
### **Description**

This change enhances the rabbitmqreceiver to collect a comprehensive set
of 74 node-level metrics from the /api/nodes endpoint.
Metrics span multiple categories, including:

- Memory usage
- Disk space and alarms
- File descriptors (usage, limits, rates)
- Socket usage and rates
- Process count and rates
- Garbage collection stats
- I/O statistics
- Context switches
- Cluster info and runtime metadata
- Plugin/configuration metadata

This provides richer observability into RabbitMQ node performance,
capacity, and system health.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
### **Link to tracking issue**
Fixes open-telemetry#38997

<!--Describe what testing was performed and which tests were added.-->
### **Testing**

- Added unit tests to verify parsing of new metrics fields.
- Manually tested against a local RabbitMQ instance with the management
plugin enabled.
- Validated metric names, types, units, and attributes emitted by the
receiver.

<!--Describe the documentation added.-->
### **Documentation**
Updated README with a reference to newly supported node metrics and
metric categories.

<!--Please delete paragraphs that you did not use before submitting.-->

Author: VenuEmmadi

.chloggen/rabbitmqreceiver_node_metrics_full_exposure.yaml

@@ -0,0 +1,11 @@
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: enhancement
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: rabbitmqreceiver
+
+# A brief description of the change. Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: "Enhance the RabbitMQ receiver to collect and report additional node-level metrics across multiple categories. These include metrics related to memory, file descriptors, sockets, processes, disk, uptime, scheduling, garbage collection (GC), I/O, message store, connections, clustering, configuration, application info, and context switches. This significantly improves visibility into the performance, state, and resource usage of RabbitMQ nodes."
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [38997]

receiver/rabbitmqreceiver/README.md

@@ -86,4 +86,13 @@ The full list of settings exposed for this receiver are documented in [config.go
 
 ## Metrics
 
-Details about the metrics produced by this receiver can be found in [metadata.yaml](./metadata.yaml)
+This receiver collects RabbitMQ node-level metrics from the `/api/nodes` endpoint. Metrics are categorized into the following groups:
+
+- **Memory usage**: `rabbitmq.node.mem_used`, `rabbitmq.node.mem_limit`, `rabbitmq.node.mem_alarm`
+- **Disk space**: `rabbitmq.node.disk_free`, `rabbitmq.node.disk_free_limit`, `rabbitmq.node.disk_free_alarm`
+- **File descriptors & sockets**: `rabbitmq.node.fd_used`, `rabbitmq.node.sockets_used`, etc.
+- **Process & scheduling**: `rabbitmq.node.proc_used`, `rabbitmq.node.run_queue`, `rabbitmq.node.context_switches`
+- **Garbage collection & I/O**: `rabbitmq.node.gc.num`, `rabbitmq.node.io_read_avg_time`, etc.
+- **Cluster & node metadata**: `rabbitmq.node.uptime`, `rabbitmq.node.processors`, etc.
+
+Details about the metrics produced by this receiver and full list of supported metrics can be found in [metadata.yaml](./metadata.yaml)