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