monitoring.md: Add input Tail plugin metrics to monitoring page

[c-neto]

This PR closes the following Issue:

• monitoring.md: Add input Tail plugin metrics to monitoring page #2379

Summary by CodeRabbit

• Documentation
  • Added detailed Tail input plugin metrics documentation covering six new metrics for file operations and line-processing (opened/closed/rotated, long-line handling, multiline truncation), including labels, descriptions, types, units, and guidance for monitoring and troubleshooting.

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📝 Walkthrough

Walkthrough

Adds documentation for six Tail input plugin metrics (files_closed, files_opened, files_rotated, long_line_skipped, long_line_truncated, multiline_truncated) to monitoring docs and adds a reference to those metrics in the Tail input plugin documentation. No code or behavioral changes.

Changes

Cohort / File(s)	Summary
Monitoring docs administration/monitoring.md	Adds a "Tail input plugin metrics" section documenting six Tail input metrics with labels, descriptions, types, and units.
Tail input docs pipeline/inputs/tail.md	Adds an "Input tail metrics" subsection referencing the new monitoring documentation for Tail input metrics.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~3 minutes

Possibly related issues

• monitoring.md: Add input Tail plugin metrics to monitoring page #2379: Adds the same Tail input plugin metrics documentation to monitoring and Tail plugin docs.

Suggested labels

4.2.1

Poem

🐰 I hopped through docs with pen in paw,

Counting files and lines I saw,
Six small metrics in a tidy row,
Tail logs tracked where numbers glow,
Hooray — the rabbit's metrics show!

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main change: adding Tail input plugin metrics documentation to the monitoring page, which aligns with the file modifications shown in the summary.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Merge Conflict Detection	✅ Passed	✅ No merge conflicts detected when merging into master

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 3

🤖 Fix all issues with AI agents

In `@administration/monitoring.md`:
- Line 236: The section heading "Tail Input Plugin Metrics" uses title-style
capitalization; change it to sentence-style by editing the heading text to "Tail
input plugin metrics" in the documentation (look for the heading string "Tail
Input Plugin Metrics") so only the first word is capitalized and the rest are
lowercased as appropriate.
- Line 247: Update the metric description for
fluentbit_input_multiline_truncated_total to fix typos: change “occurences” to
“occurrences” and “multilines” to “multiline” so the description reads something
like “Total number of truncated occurrences for multiline. This metric is only
created when `multiline.parser` is defined.” Ensure you modify the line
containing the metric name `fluentbit_input_multiline_truncated_total` in
monitoring.md.

In `@pipeline/inputs/tail.md`:
- Around line 573-575: The heading "### Input Tail Metrics" uses title-style
capitalization; change it to sentence-style capitalization by updating the
heading text (look for the literal "### Input Tail Metrics") to "### Input tail
metrics" so only the first word is capitalized and plugin name is lowercased per
house style.

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@pipeline/inputs/tail.md`:
- Around line 573-575: The markdown under the "Input tail metrics" section uses
an absolute link to /administration/monitoring.md which can break site routing;
update the link to the relative path used elsewhere in this doc (e.g.,
../../administration/monitoring.md) so the sentence "For detailed information
about available metrics and how to use them, see [Monitoring](...)." points to
the relative location instead of the absolute root.

[eschabell]

Hi @c-neto,

Thanks for the fixes you are suggesting here. I have unfortunately just before you sent these submitted a rather comprehensive update to the Monitoring doc page. The changes you propose I have pulled into that PR and tagged it as fixing this issue, so once it merges, it should close this out for you.

I will be marking your doc PR here as duplicate, to save us both time as the location you wanted to add them is I believe incorrect. I have added them into the existing table in this PR #2378 while giving you named credit for the help in the commit.

I have cross-posted this to your original issue.