docs: installation: upgrade-notes: document 5.1 input rate metrics and rate gate

[eschabell]

Add an upgrade-notes entry for the new input rate_window/rate_gate
settings and fluentbit_input_rate_* metrics, which landed in
administration and pipeline docs but weren't cross-referenced in the
v5.1 upgrade notes. Also reword the mutual TLS note to clear a Vale
WordList suggestion on "terminate".

Summary by CodeRabbit

• Documentation
  • Added guidance for per-input ingestion-rate metrics in Fluent Bit v5.1.
  • Documented the optional rate gate, including configurable thresholds, backpressure, and resume behavior.
  • Clarified when tls.verify_client_cert applies during TLS termination.

[coderabbitai]

📝 Walkthrough

Walkthrough

The upgrade notes now describe Fluent Bit v5.1 input rate metrics, opt-in rate gating, and clarified mTLS termination behavior.

Changes

Upgrade notes updates

Layer / File(s)	Summary
Document v5.1 upgrade behavior installation/upgrade-notes.md	Adds input byte and record rate metrics, rate_gate controls, configurable thresholds, backpressure, and resume behavior. Clarifies that Fluent Bit terminates TLS when tls.verify_client_cert uses certificate and key files.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Possibly related PRs

• fluent/fluent-bit-docs#2637: Documents the same input rate metrics and rate-gate features.
• fluent/fluent-bit-docs#2664: Documents related Fluent Bit v5.1 rate-gating settings and behavior.
• fluent/fluent-bit-docs#2666: Updates the same upgrade-notes document for Fluent Bit v5.1.

Suggested reviewers: patrick-stephens, cosmo0920

🚥 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 clearly and concisely identifies the v5.1 upgrade-notes documentation for input rate metrics and the rate gate.
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.

✨ Finishing Touches

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@installation/upgrade-notes.md`:
- Line 23: Update the v5.1 rate metrics and rate limiting note to document the
per-input rate_window setting, stating that it controls the byte and record
measurement window and defaults to 1s, consistent with the existing pipeline
buffering documentation.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

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• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: c979f8fc-0a56-4593-bbb5-94a1fd1c1821

📥 Commits

Reviewing files that changed from the base of the PR and between 4087ee3 and 28cbc6b.

📒 Files selected for processing (1)

• installation/upgrade-notes.md

[eschabell]

@patrick-stephens ready for review!