Report time to first content in chat reducer sample

[ilia-sokolov]

Motivation and Context

The streaming chat-history reducer sample reports aggregate token usage but gives no indication of the latency users experience before the first response content becomes available.

This change addresses #9580 by making that latency visible for the reducer-enabled streaming path. The label explicitly states that chat-history reduction is included, avoiding the misleading implication that the value represents provider-only latency.

Description

• start a per-turn stopwatch immediately before requesting the streaming response
• stop and report it once, on the first non-empty textual update
• preserve the existing response assembly and token accounting
• report a truthful fallback when a stream returns no text, such as a metadata-only or tool-only stream

The change is confined to MultipleProviders_ChatHistoryReducer.cs, introduces no dependencies or public APIs, and intentionally avoids timing thresholds because provider latency is nondeterministic.

Validation

The exact modified sample source compiled successfully with 0 warnings and 0 errors against the released Semantic Kernel 1.78 packages.

The repository-wide Concepts build and formatter traverse a large connector project graph and did not complete in the available validation window. The modified Semantic Kernel API expressions are unchanged from main; the added implementation uses only System.Diagnostics.Stopwatch and local control flow.

Contribution Checklist

• The complete repository build finishes without any errors or warnings
• The repository-wide pre-submission formatting script raises no violations
• All repository unit tests pass
• The focused sample change compiles cleanly and preserves existing behavior
• I didn't break anyone 😄

[github-actions]

Automated Code Review

Reviewers: 5 | Confidence: 92% | Result: All clear

Reviewed: Correctness, Security Reliability, Test Coverage, Failure Modes, Design Approach

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Automated review by ilia-sokolov's agents

[Copilot]

Pull request overview

Adds a latency metric to the .NET streaming chat-history reducer sample so users can see how long it takes before the first assistant text becomes available when chat-history reduction is enabled.

Changes:

• Start a per-turn Stopwatch before initiating streaming and print elapsed time on the first non-empty text update.
• Preserve existing response assembly and token usage aggregation while streaming.
• Add a fallback message for streams that produce no text content.

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