Support `ros2 param get <parameter>` across all nodes

[fujitatomoya]

Description

Currently, ros2 param get requires both a node name and parameter name as positional arguments:

ros2 param get <node_name> <parameter_name>

This proposal suggests extending the command to support an optional node name, allowing users to query a parameter across all discovered nodes:

ros2 param get <parameter_name>           # Query all nodes
ros2 param get <node_name> <parameter_name>  # Existing behavior (unchanged)

When invoked without a node name, the command would iterate through all available nodes and print the parameter value (or indicate if not set) for each.

Motivation

System-wide parameter inspection: Parameters like use_sim_time are commonly set across many nodes. Verifying consistency currently requires running multiple commands or writing a script.
Debugging efficiency: When troubleshooting parameter-related issues, developers often need to check the same parameter across the entire system to identify misconfigurations.

Design / Implementation Considerations

Proposed Syntax

# New: Query parameter across all nodes
ros2 param get use_sim_time

# Existing: Query specific node (unchanged)
ros2 param get /my_node use_sim_time

Output Format

/node_a:   use_sim_time: False
/node_b:   use_sim_time: True
(/node_c:   if parameter not set, it does not print)

• Fully backward compatible
• Existing ros2 param get <node> <param> syntax unchanged
• Disambiguation: If ambiguity exists between a node name and parameter name, prefer the existing 2-argument interpretation or require explicit --all-nodes flag

Additional Information

This pattern could extend to other ros2 param subcommands:

• ros2 param set <parameter> <value> — Set across all nodes (with confirmation)