Add Application Insights telemetry example

[lntutor]

Summary

• add the Azure Monitor OpenTelemetry distro to the compiled ASP.NET Core MCP server sample
• select Application Insights when APPLICATIONINSIGHTS_CONNECTION_STRING is configured while retaining OTLP as the default
• document setup, exported signals, MCP attributes, and a sample Application Insights query

Closes #1026

Test plan

• dotnet build samples/AspNetCoreMcpServer/AspNetCoreMcpServer.csproj --configuration Debug --no-incremental
• dotnet build samples/AspNetCoreMcpServer/AspNetCoreMcpServer.csproj --configuration Release --no-incremental
• OTLP-default startup smoke test and MCP endpoint request
• Azure Monitor startup smoke test with a non-secret placeholder connection string
• dotnet build ModelContextProtocol.slnx --configuration Debug --no-incremental
• dotnet test ModelContextProtocol.slnx --configuration Debug --no-build --no-restore --filter "(Execution!=Manual)" (8,862 passed)
• focused dotnet format verification for the modified C# file; the repository-wide formatter currently reports 74 pre-existing files on main

[lntutor]

Current head 780b835370116b01cebce0cc3c592005f8e4f567 is mergeable and the visible CI surface on this exact SHA is fully green: markdown-link-check, CodeQL (Analyze (actions) and Analyze (csharp)), all Debug/Release build-and-test jobs on Ubuntu, macOS, and Windows, publish-coverage, and the aggregate CodeQL check all completed successfully. There are no review threads or review-body comments open on the current PR state, so if this Application Insights telemetry example still matches the intended direction, it looks ready for maintainer review/merge when convenient.

[jeffhandley]

Great addition here, @lntutor; thanks!