Docs for SERVER-35300: Track total transaction duration in TxnStats

Description

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Engineering Ticket Description:

We want to track total transaction duration in TxnStats. To do this, we can add a TxnStats::startTime field. When a transaction begins, we will save the start time of the transaction. This will allow us to compute the total elapsed time of the transaction as (now() - startTime), when the transaction commits or aborts.

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