fix: Add missing Build()

[hangy]

The suggested OpenTelemetry SDK methods return a …Builder instance. Thus, to use the TracerProvider, or the MeterProvider, they need to be built.

[prmerger-automator]

@hangy : Thanks for your contribution! The author(s) have been notified to review your proposed change.

[learn-build-service-prod]

Learn Build status updates of commit 8f5e644:

✅ Validation status: passed

File	Status	Preview URL	Details
articles/azure-monitor/app/opentelemetry-configuration.md	✅Succeeded

For more details, please refer to the build report.

For any questions, please:

• Try searching the learn.microsoft.com contributor guides
• Post your question in the Learn support channel

[v-dirichards]

@AaronMaxwell

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

#label:"aq-pr-triaged"
@MicrosoftDocs/public-repo-pr-review-team

[AaronMaxwell]

@TimothyMothra - We have a request to modify the .NET sample code with some additional build statements. Please take a look and let me know if this should be accepted, rejected, or modified.

[AaronMaxwell]

#assign-reviewer:TimothyMothra

[TimothyMothra]

Thanks for fixing this!
Don't know how this was missed. It's in all our other docs and examples.

[v-dirichards]

@TimothyMothra I'll merge this PR based on your approval. If this article requires further changes, please open a separate pull request.