https://github.com/mongodb/mongo/blob/f3a8b79683137be0f8256358a4d424fa88ebfda7/src/mongo/executor/connection_pool.cpp#L1115

The timeout that ends up being logged is not the timeout of the request (ie, the total time request spent) but it appears to be the time in between the last time ConnectionPool::SpecificPool::updateEventTimer was called and the request with the earliest expiration.