Use ETW events

[buybackoff]

There are many examples how to use ETW in System.Buffer, in RecyclableMemoryStream, etc. It is quite easy and more appropriate for performance monitoring than tracing or logging. Profilers support such events quite well. This will be relevant when real-world big integration tests are done.

However, since we are already using the mentioned libraries and the main enemy is needless allocations that are visible even without any ETW, this is a nicety for now. Should add them only when there are tweakable configs that could have double-digit+ percentage performance impact.