Add org.matrix.msgid to to device messages for easier tracking

[krille-chan]

Preflight Checklist

• I could not find a solution in the existing issues, docs, nor discussions

Describe your problem

Original idea from @kegsay

I've been debugging why one Element X user cannot decrypt a message. Logs suggest that the to-device message containing the new room key was never received. This could be because:

The sender never sent it
The server never delivered it

Unfortunately, I cannot verify either of these. The sender was on FluffyChat. Is there any way I can rule out FluffyChat as a cause? Element clients tag to-device messages with an extra key under content called org.matrix.msgid which can then be logged server-side, enabling the message to be traced across multiple services. If FluffyChat did the same, and also provided some way to expose the logs client-side for debugging, then it would be possible to see if the client did send the room key and if the server got it / delivered it.

Describe your ideal solution

Add a org.matrix.msgid to to_device messages with a unique transaction ID to track events server-wide.

Version

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Security requirements

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Additional Context

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