Docs for SERVER-35294: Add total number of open transactions to serverStatus

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Original Description

Description:

Updates to serverStatus.

Engineering Ticket Description:

We want to output the total number of open transactions in the "transactions" serverStatus section by utilizing the existing metrics tracked in ServerTransactionsMetrics::currentOpen. We need to add a currentOpen field to the transactions_stats.idl class, so that we can serialize the counter to the serverStatus output object in the TransactionsSSS::generateSection method. The TransactionsSSS::generateSection method returns a BSON object that goes into the transactions section of the serverStatus output.

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Description

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Impact to other docs outside of this product

MVP (work and date?)

Resources (e.g. Scope Docs, Invision)