Resolve #1614: Correct metrics book capability labels

[ayden94]

Summary

• Aligns the beginner metrics chapter with the current @fluojs/metrics contract.
• Corrects tutorial overclaims around automatic infrastructure labels and Summary support.

Closes #1614

Changes

• Updates EN/KO beginner metrics book text to state that built-in platform telemetry supports documented env and instance labels only.
• Replaces the Summary section with the supported custom metric surface: Counter, Gauge, and Histogram.
• Adds http_errors_total to the beginner chapter's HTTP instrumentation list so it matches the package README contract.
• Changeset decision: no changeset. This is book-only documentation correction with no package source, public API, runtime behavior, or published package artifact impact.

Testing

• LSP diagnostics: clean for book/beginner/ch19-metrics.md and book/beginner/ch19-metrics.ko.md.
• pnpm lint (completed; existing repository-wide Biome warnings/infos remain outside these changed book files, and verify:public-export-tsdoc passed in changed mode).

Public export documentation

• No public exports changed.
• Changed exported functions document matching @param / @returns tags where applicable. N/A.
• Source @example blocks and README scenario examples still play complementary roles. N/A.

Behavioral contract

• No documented behavioral contracts were removed without migration notes.
• New behavioral contracts are documented in the affected package README. N/A; this PR aligns book text to the existing package README contract.
• Intentional limitations are explicitly stated (not silently removed).
• Runtime invariants are covered by regression tests. N/A; documentation-only correction.

Platform consistency governance (SSOT)

• SSOT English/Korean mirror structure remains synchronized for changed governance docs. N/A; changed EN/KO book mirrors remain aligned.
• If platform contract docs changed, companion updates include discoverability/docs index, tooling or CI enforcement, and regression-test evidence. N/A.
• Any package README alignment/conformance claims are backed by createPlatformConformanceHarness(...) tests. N/A.

Residual risks

• pnpm lint reports pre-existing repository-wide Biome warnings/infos outside this PR's changed book files; they were not modified here.