Investigate changes in SERVER-54278: Add feature flag to disable Opportunistic Reads by default

Description

Description of Linked Ticket

Opportunistic reads against replica-sets was originally introduced in v4.2 as part of PM-856. This feature allows mongos to direct secondary reads to any secondary with an active connection, instead of waiting for a new connection to a specific secondary.

Under certain workloads, as described in SERVER-53853, opportunistic reads may trigger construction of unnecessary connections to mongod processes, which would regress the overall performance.

This ticket should introduce a new feature flag to disable opportunistic reads in v4.2 by default. In particular, the following tickets from PM-856 introduce new functionality that we might want to guard with the feature flag: SERVER-41132, SERVER-41133, and SERVER-41134.

We may wan to approach this ticket from this for loop, introduced by SERVER-41133 (see excerpt below).

// Attempt to get a connection to every target host

for (size_t idx = 0; idx < request.target.size() && !cmdState->finishLine.isReady(); ++idx) {

    auto connFuture = _pool->get(request.target[idx], request.sslMode, request.timeout);

    ...

}

Scope of changes

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MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)