feat(docs): document Metering & Billing plugin event dimensions

[gergely-kurucz-konghq]

Description

What

• Adds a Captured event dimensions section to the Metering & Billing plugin page (## How it works), with a table per emitted event type (kong.api_request, kong.llm_request) plus the conditional portal/application fields. Rendered from a new _data file via an include.
• Links each guide's "Create a meter" step to that reference.

Why

The plugin page said it "captures standard Kong Gateway metadata" but never listed the fields it emits — that dimension catalog was undocumented. The how-to guides show only a curated subset, so readers had nowhere to find the complete set.

How

Field lists are transcribed from the plugin source cloudevent.lua (create_api_event / create_ai_event). The table is data-driven (app/_data/plugins/metering-and-billing.yaml + app/_includes/plugins/metering-and-billing/event_dimensions.md), following the existing OTel metric_tables.md pattern. Operator-configured custom attributes are documented as an extension mechanism (they can't be enumerated from source). Render-checked locally: both event tables + portal table render, and both guide links resolve to #captured-event-dimensions.

Fixes https://konghq.atlassian.net/browse/OM-433

Preview Links

• /plugins/metering-and-billing/#captured-event-dimensions
• /metering-and-billing/get-started/#create-a-meter
• /how-to/meter-llm-traffic/#create-a-meter

Checklist

• Tested how-to docs. Render-verified the plugin page and both guide links locally.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable). — N/A, no new pages (a section on an existing page).

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