session.command event hits a ConditionalCheckFailedException on dynamoevents

[espadolini]

Expected behavior:
session.command events are stored in the audit log even when using the DynamoDB events backend.

Current behavior:
sesssion.command events fail to get stored, reporting a ConditionalCheckFailedException error from DynamoDB.

Bug details:
All session.command events are emitted with the session ID of the session they belong to, and an event index of 0. DynamoDB uses (session ID, event index) as a primary key for the event, so prior to #38495 each session.command event overwrote the initial session.start event at first, and then all the other session.commands that took its place; now we refuse to overwrite events, rejecting the event and storing an error.

A similar issue exists for a different event (#39833), suggesting that perhaps we should fix the problem in a more general way rather than chase individual instances of event indexes not being set correctly for a session event.

• Teleport version: 15.2.0, but it's likely that we've only just noticed now that we're logging errors rather than overwriting.