[MTP] Add initial support for OTel

[Evangelink]

This PR introduces OpenTelemetry support to the platform, enabling distributed tracing and metrics collection for test runs. The main changes include adding a new extension project for OpenTelemetry integration, updating dependencies and sample usage, and instrumenting the test execution flow to emit telemetry data.

• Added new project Microsoft.Testing.Extensions.OpenTelemetry with wrappers for activities, counters, and histograms, and a platform service implementation for telemetry instrumentation.

• Modified Playground.csproj and Program.cs to demonstrate enabling OpenTelemetry tracing and metrics via the new provider and exporter.

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[Copilot]

Pull Request Overview

This PR introduces OpenTelemetry support to the Microsoft Testing Platform, enabling distributed tracing and metrics collection for test runs. The implementation provides instrumentation capabilities while maintaining compatibility with the platform's netstandard2.0 target framework.

Key changes include:

• Added new Microsoft.Testing.Extensions.OpenTelemetry extension project with OpenTelemetry integration
• Introduced telemetry interfaces and platform services for activity tracking, counters, and histograms
• Updated existing services to accept the new OpenTelemetry service parameter and instrument test execution flows
• Enhanced the Playground sample to demonstrate OpenTelemetry usage with OTLP exporters

Reviewed Changes

Copilot reviewed 40 out of 40 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/Telemetry/TelemetryManager.cs	Added OpenTelemetry provider support and renamed BuildAsync to BuildTelemetryAsync
src/Platform/Microsoft.Testing.Platform/Services/TestApplicationResult.cs	Added OpenTelemetry service integration and test execution instrumentation
src/Platform/Microsoft.Testing.Extensions.OpenTelemetry/	New extension project providing OpenTelemetry wrappers and platform service implementation
test/UnitTests/	Updated test constructors to accommodate new OpenTelemetry service parameter
src/Platform/Microsoft.Testing.Platform/Hosts/	Added activity tracking throughout host execution flows
samples/Playground/	Added OpenTelemetry configuration example with tracing and metrics