[AzureMonitorExporter] Add OTEL_TRACES_SAMPLER support to Exporter and AspNetCore distro

[rajkumar-rangaraj]

• Added support for OTEL_TRACES_SAMPLER / OTEL_TRACES_SAMPLER_ARG (microsoft.rate_limited, microsoft.fixed_percentage) in:
  • DefaultAzureMonitorExporterOptions and direct AddAzureMonitorTraceExporter path
  • DefaultAzureMonitorOptions (AspNetCore distro)
• Added unit tests for exporter and AspNetCore options + tracer builder path

TODO

• Move common logic to utility class and share between 2 projects.

[Copilot]

Pull Request Overview

This PR adds support for configuring Azure Monitor sampling through OpenTelemetry environment variables OTEL_TRACES_SAMPLER and OTEL_TRACES_SAMPLER_ARG. The feature allows users to specify either Microsoft's rate-limited or fixed-percentage samplers through standard OpenTelemetry configuration patterns.

• Added support for microsoft.rate_limited and microsoft.fixed_percentage sampler types
• Implemented configuration precedence: environment variables override IConfiguration values
• Added comprehensive unit tests for both exporter and AspNetCore distribution paths

Reviewed Changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
DefaultAzureMonitorExporterOptionsTests.cs	New test file validating sampler configuration for the exporter options
AddAzureMonitorTraceExporterSamplerTests.cs	New test file verifying sampler integration with TracerProviderBuilder
EnvironmentVariableConstants.cs	Added constants for the new OpenTelemetry sampler environment variables
AzureMonitorExporterEventSource.cs	Added logging events for invalid sampler configuration
DefaultAzureMonitorExporterOptions.cs	Implemented sampler configuration logic for the exporter
AzureMonitorExporterExtensions.cs	Updated extension methods to support default options configuration
DefaultAzureMonitorOptionsSamplerTests.cs	New test file for AspNetCore sampler configuration
DefaultAzureMonitorOptions.cs	Implemented sampler configuration logic for AspNetCore
AzureMonitorAspNetCoreEventSource.cs	Added logging events for AspNetCore sampler validation
Both CHANGELOG.md files	Documented the new sampler configuration feature

[harsimar]

This looks ok for the most part, just a minor non-blocking comment