crypto: Include megolm ratchet index in logging span fields

[richvdh]

This field is helpful as it tells us the sequence number of the message in the megolm session, which gives us a clue about how long it will have been since the session should have been shared with us.

[Question: an alternative would be just to attach the whole of content to the span, instead of separate fields for sender_key, session_id, message_index. The Debug defintion for MegolmV1AesSha2Content looks like it contains the right things, so the code might be simpler. On the other hand, I think the output would be less readable. Thoughts?]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.32%. Comparing base (2408df8) to head (6901efe).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3989      +/-   ##
==========================================
+ Coverage   84.27%   84.32%   +0.05%     
==========================================
  Files         266      266              
  Lines       28296    28299       +3     
==========================================
+ Hits        23847    23864      +17     
+ Misses       4449     4435      -14     

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[poljar]

[Question: an alternative would be just to attach the whole of content to the span, instead of separate fields for sender_key, session_id, message_index. The Debug defintion for MegolmV1AesSha2Content looks like it contains the right things, so the code might be simpler. On the other hand, I think the output would be less readable. Thoughts?]

I think I prefer separate fields, since the content might contain unknown fields somebody might include something they might not want to end up in logs or might contain spam.