docs: document oauth2.user_agent for OAuth2 token requests

[rja5]

Add oauth2.user_agent documentation for outgoing OAuth 2.0 token requests.

Summary by CodeRabbit

• Documentation
  • Added docs for the oauth2.user_agent option so HTTP and OpenTelemetry outputs can include an optional User-Agent header on OAuth 2.0 token requests.
  • Clarified that omitting or setting the option to _none_ results in no User-Agent header being sent.
  • Updated pipeline docs to describe this shared oauth2.user_agent behavior.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a560a55d-8929-41bd-894e-7a53ff862764

📥 Commits

Reviewing files that changed from the base of the PR and between c13fbe8 and ee7d0eb.

📒 Files selected for processing (1)

• administration/configuring-fluent-bit/yaml/pipeline-section.md

✅ Files skipped from review due to trivial changes (1)

• administration/configuring-fluent-bit/yaml/pipeline-section.md

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📝 Walkthrough

Walkthrough

Adds documentation for a new oauth2.user_agent option that controls an optional User-Agent header sent with OAuth 2.0 client-credentials token requests; updates the pipeline overview, HTTP output, and OpenTelemetry output docs.

Changes

OAuth 2.0 User-Agent Option Documentation

Layer / File(s)	Summary
OAuth 2.0 user agent option across output plugins administration/configuring-fluent-bit/yaml/pipeline-section.md, pipeline/outputs/http.md, pipeline/outputs/opentelemetry.md	Adds the oauth2.user_agent documentation row to the pipeline outputs table and documents the option in the HTTP and OpenTelemetry output guides, describing when a User-Agent header is included in OAuth 2.0 token requests.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• fluent/fluent-bit-docs#2434: Overlaps with HTTP output oauth2.* documentation updates.
• fluent/fluent-bit-docs#2489: Related OpenTelemetry output OAuth2 documentation changes.

Suggested labels

5.0

Suggested reviewers

• cosmo0920
• patrick-stephens

Poem

🐰 I nibble lines and tidy docs with care,
A User-Agent now hums on token calls there,
Pipeline, HTTP, Telemetry all in a row,
A tiny header, optional and low,
I hop to the commit and give it a glow.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and clearly describes the main change: documenting the oauth2.user_agent configuration option for OAuth 2.0 token requests, which aligns with all three file modifications in the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[eschabell]

@rja5 one back-tick fix for a vale issue and it's good to go, please fix.

[eschabell]

@rja5 use back-ticks around the OAuth 2.0 to fix this vale issue.