Limit StructValue resize allocations

[clutterloh]

We use TraceEvent to gather process-level event counters. I noticed in a recent trace that about 12% of our process memory allocations are due to resizing when we call DynamicTraceEventData.PayloadValue(int32)

In particular, the system.private.corelib.il!System.Collections.Generic.List`1[System.Collections.Generic.KeyValuePair`2[System.__Canon, System.__Canon]].AddWithResize(System.__Canon) and system.private.corelib.il!System.Collections.Generic.List`1[System.Collections.Generic.KeyValuePair`2[System.__Canon, System.__Canon]].set_Capacity(int32) frames caused by list resizing.

I noticed in the code that when GetPayloadValueAt(...) is called a StructValue is created and we go through a loop adding i KeyValuePairs corresponding to classInfo.FieldFetches.Length.
https://github.com/microsoft/perfview/blob/4685da655131acb73441f71ec19ea79ffa300ffa/src/TraceEvent/DynamicTraceEventParser.cs#L606C17-L612C18

[brianrob]

Thanks for contributing this @clutterloh. This seems like a completely reasonable change that could have some good benefit.