go: fix channel auto coalescing in merge

[bradsquicciarini-coco]

Changelog

Ensure channel auto coalescing is deterministic in mcap merge

Docs

Description

In the mcap merge command, the channel auto coalescing is currently non-deterministic due to the random ordering of iterating over a map in go. This change just sorts the metadata keys before we hash it to ensure for the same channel metadata we always get the same hash.

For example, if trying to merge these to mcaps

library:   mcap go v1.7.1; mcap go v1.7.0
profile:
messages:  800
duration:  19.979441843s
start:     2024-07-01T10:23:52.441156385-07:00 (1719854632.441156385)
end:       2024-07-01T10:24:12.420598228-07:00 (1719854652.420598228)
compression:
        zstd: [1/1 chunks] [273.00 KiB/36.78 KiB (86.53%)] [1.84 KiB/sec]
channels:
        (4) /imu  800 msgs (40.04 Hz)   : sensor_msgs/Imu [ros1msg]
channels: 1
attachments: 0
metadata: 0

library:   mcap go v1.7.1; mcap go v1.7.0
profile:
messages:  800
duration:  19.979921296s
start:     2024-07-01T15:05:18.571434739-07:00 (1719871518.571434739)
end:       2024-07-01T15:05:38.551356035-07:00 (1719871538.551356035)
compression:
        zstd: [1/1 chunks] [273.00 KiB/35.08 KiB (87.15%)] [1.75 KiB/sec]
channels:
        (4) /imu  800 msgs (40.04 Hz)   : sensor_msgs/Imu [ros1msg]
channels: 1
attachments: 0
metadata: 0

will sometimes give this

$ mcap merge imu0.mcap imu1.mcap -o test.mcap && mcap info test.mcap
library:   mcap go v1.7.1
profile:
messages:  1600
duration:  4h41m46.11019965s
start:     2024-07-01T10:23:52.441156385-07:00 (1719854632.441156385)
end:       2024-07-01T15:05:38.551356035-07:00 (1719871538.551356035)
compression:
        zstd: [1/1 chunks] [543.32 KiB/67.37 KiB (87.60%)] [4.00 B/sec]
channels:
        (1) /imu  1600 msgs (0.09 Hz)   : sensor_msgs/Imu [ros1msg]
channels: 1
attachments: 0
metadata: 0

and sometimes give this

mcap merge imu0.mcap imu1.mcap -o test.mcap && mcap info test.mcap
library:   mcap go v1.7.1
profile:
messages:  1600
duration:  4h41m46.11019965s
start:     2024-07-01T10:23:52.441156385-07:00 (1719854632.441156385)
end:       2024-07-01T15:05:38.551356035-07:00 (1719871538.551356035)
compression:
        zstd: [1/1 chunks] [543.45 KiB/67.85 KiB (87.51%)] [4.00 B/sec]
channels:
        (1) /imu  800 msgs (0.05 Hz)   : sensor_msgs/Imu [ros1msg]
        (2) /imu  800 msgs (0.05 Hz)   : sensor_msgs/Imu [ros1msg]
channels: 2
attachments: 0
metadata: 0

I've updated the unit tests to test for this.

Fixes: foxglove/repo#772