ThreadsafeCounterDictionary allocates a CounterItem on every recorded drop #5686

Part of #5634.

Branch: main. No public API change, so this doesn't need to wait for v7.

ThreadsafeCounterDictionary.Add and Increment call _items.GetOrAdd(key, new CounterItem()) (ThreadsafeCounterDictionary.cs:27,33). That allocates a CounterItem on every call, even when the key already exists. #5607 adds call sites on hot paths, such as every dropped log or metric.

Measured: a dropped log went from 0 B to 24 B allocated. The factory overload, GetOrAdd(key, static _ => new CounterItem()), brings it back to 0 B. CollectionsExtensions.cs:10 already uses the factory overload.