Backport(v1.19): fix(metrics): export-only clamp for buffer statistics (#5467)

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Which issue(s) this PR fixes:
Backport #5467
Fixes #5303

What this PR does / why we need it:
Buffer size gauges (stage_byte_size, queue_byte_size, and derived total_queued_size) can go transiently negative when Fluentd core under/over-subtracts during concurrent stage/queue transitions (the deferred @stage_size_metrics.add after chunk unlock vs enqueue_chunk's sub — see #2712 / #2734). Those values are mirrored by the Prometheus plugin as fluentd_output_status_buffer_total_bytes / fluentd_output_status_buffer_stage_byte_size, which is what #5303 reports.

Clamping the gauge store on sub/dec is the wrong fix: it turns a self-correcting transient negative into a permanent over-count, so Buffer#storable? (which reads the raw gauge) eventually refuses every write. Thanks @Watson1978 for catching that.

This PR takes an export-only approach:

1. Leave LocalMetrics gauge sub/dec/set semantics unchanged (negatives still allowed so the deferred-add race can self-heal).
2. Clamp stage/queue sizes to >= 0 only when building statistics (the path Prometheus and the monitor agent consume).
3. Clamp available_buffer_space_ratios to [0, 100] when counters overshoot total_limit_size, and treat total_limit_size == 0 as 0% free without dividing (no NaN / no dead NaN guard).

Docs Changes:
None

Release Note:

• buffer: clamp exported buffer size metrics to non-negative values

General Checklist:

• I have read and followed CONTRIBUTING.md
• tests pass (CI)
• DCO signed-off

Tests:

• test/plugin/test_buffer.rb #statistics: negative underlying gauges export as 0; overshoot clamps ratio to 0; total_limit_size == 0 does not raise and ratio stays finite.