feat(landing): Prometheus metrics guide - #1567

🟢 Confidence 10/10 · safe to merge
Static Astro landing page; the highlighter escapes before set:html, the bucket math and links check out, and nothing else in the diff mutates state.
_{no findings · tests not needed · risk low}

Adds a static /guides/prometheus-metrics landing guide, a small guide-specific syntax highlighter, and its entries in the guides index and llms.txt. Contained and safe to merge.

• guide-highlight.ts tokenizes exposition, PromQL, YAML and six client languages for set:html code blocks
• Guide page composes bucket, type and client tables from page-local constants
• Guides index and llms.txt list the new guide

Production impact

_{d46eabc · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}