.NET: Improve observability sample

[TaoChenOSU]

Motivation and Context

Update the Agent Observability sample .Net based on feedback from office hour.

Description

1. Remove confusing source name from AddSource() as it's not necessary.
2. Doc update follows in the doc repo.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

Updates the .NET AgentOpenTelemetry sample to simplify OpenTelemetry configuration and reduce confusion around instrument/source registration.

Changes:

• Removed wildcard AddSource / AddMeter registrations intended for Agent Framework telemetry.
• Tweaked the sample’s SourceName documentation and clarified UseOpenTelemetry usage with a named argument.