.Net: Add metric for time to first token for the chat history reducer samples

[markwallace-microsoft]

[ilia-sokolov]

I’ve prepared a focused .NET sample update for this issue. It measures reducer-inclusive time to first non-empty textual content in MultipleProviders_ChatHistoryReducer, reports a truthful fallback for streams with no text, and preserves the existing token accounting. The change is limited to the streaming reducer sample and introduces no new dependencies or public APIs. I’ll open it as a draft PR with the validation details so the approach can be reviewed before it is marked ready.