go/cli: mcap info show channels count

[pezy]

Public-Facing Changes

show channels count like channels(45): in mcap info print.

Description

The channels list printed by mcap info might have discontinuous sequence numbers due to certain reasons (for example, if one of the topics in the original recorded data has zero messages, then the mcap generated after using mcap filter will exhibit this). Therefore, it's important to visually see the number of channels.

[wkalt]

this seems nice to me but the count formatting is inconsistent with the way other counts are displayed. Here is the existing info:

library:   mcap go v0.4.0                                              
profile:   ros1                                                        
messages:  1606                                                        
duration:  7.780758504s                                                
start:     2017-03-21T19:26:20.103843113-07:00 (1490149580.103843113)  
end:       2017-03-21T19:26:27.884601617-07:00 (1490149587.884601617)  
compression:
        zstd: [314/314 chunks] [119.10 MiB/58.57 MiB (50.82%)] [7.53 MiB/sec] 
channels:
        (0) /diagnostics              52 msgs (6.68 Hz)    : diagnostic_msgs/DiagnosticArray [ros1msg]  
        (1) /image_color/compressed  234 msgs (30.07 Hz)   : sensor_msgs/CompressedImage [ros1msg]      
        (2) /tf                      774 msgs (99.48 Hz)   : tf2_msgs/TFMessage [ros1msg]               
        (3) /radar/points            156 msgs (20.05 Hz)   : sensor_msgs/PointCloud2 [ros1msg]          
        (4) /radar/range             156 msgs (20.05 Hz)   : sensor_msgs/Range [ros1msg]                
        (5) /radar/tracks            156 msgs (20.05 Hz)   : radar_driver/RadarTracks [ros1msg]         
        (6) /velodyne_points          78 msgs (10.02 Hz)   : sensor_msgs/PointCloud2 [ros1msg]          
attachments: 0
metadata: 0

what about instead adding a "channels: 7" above where "attachments: 0" is printed?

[defunctzombie]

@james-rms please take a look

[james-rms]

Closing in favor of #1243