Fix missing metrics due to incorrect handling of "Telemetry sampled out" responses

[Copilot]

Issue

Users experience missing metrics in Azure.Monitor.OpenTelemetry.AspNetCore when Application Insights sampling is enabled. The exporter logs errors like:

Received a partial success from ingestion. This status code is not handled and telemetry will be lost. Error StatusCode: {206}. Error Message: {Telemetry sampled out.}

This error occurs because the Azure Monitor OpenTelemetry exporter treats 206 Partial Success responses as failures when they include "Telemetry sampled out" messages, which are actually normal responses when sampling is enabled.

Changes

This PR makes three key changes to correctly handle sampled out telemetry:

1. Added a helper method IsSamplingError() to identify when an error is due to normal sampling behavior
2. Updated the IsSuccess() method to consider 206 (PartialSuccess) responses as successful
3. Modified error handling to skip retry and warning logs for "Telemetry sampled out" messages

Test Results

When used with an application that has ingestion sampling enabled (like the 12.5% mentioned in the issue), metrics should now appear consistently in Application Insights without interruption due to sampling.

Customer Impact

This issue affects customers using metrics with Azure.Monitor.OpenTelemetry.AspNetCore when sampling is enabled in their Application Insights instance, causing inconsistent metrics data in dashboards and alerts.

Fixes #48141.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• 4myvsblobprodcus32.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)
• 51yvsblobprodcus36.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)
• 5rqvsblobprodcus385.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)
• gbsvsblobprodcus365.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)
• nn8vsblobprodcus340.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)
• pe3vsblobprodcus354.vsblob.vsassets.io
  • Triggering command: dotnet build sdk/monitor/Azure.Monitor.OpenTelemetry.Exporter/src (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to my firewall allow list

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