In SERVER-106630, we added an additional mutex to ObserableMutexRegistry in order to allow mutexes to be registered independently of collection.

We can provide even better isolation by using a std::list as a queue between add() and _collectStats().

Evaluate whether this is worth doing.

Here's some pseudocode:

mutex _registration;    
mutex _collection;    
    
std::list<NewEntry> _new;    
some_map_thing<Entry> _entries;    
    
add() {    
    list<NewEntry> newNode{...};    
    lock{_registration};    
    _new.splice(...);    
}    
    
collect() {                      
    list<NewEntries> newbies;
    {                           
        lock{_registration};     
        newbies.splice(_new);

         }
                          
    lock{_collection};              
    for each newbie in newbies {                            
        add an Entry to _entries corresponding to newbie
         }
                                     
    // now _entries is up to date                         
    // continue with "old" implementation of collect()
}