fix(server): preserve integer metrics precision

[esko]

CACHE-002 completed all 36 requests but exact metric reconciliation failed: direct prompt timings summed to 1,423,152 while /metrics emitted 1,423,150 because integer JSON values were coerced to double and rendered with default stream precision.

This change preserves signed and unsigned integer JSON types and emits exact decimal literals. Finite floating metrics use round-trip-safe precision; nonnumeric and nonfinite values fail safely. HELP/TYPE/sample layout and metric names remain unchanged.

Validation:

• Release + Metal + embedded Metal llama-server build passed
• focused formatter test passed
• relevant CTest selection passed 4/4
• bounded /metrics endpoint smoke passed

Correctness-only change; no performance claim. Do not merge until deployment repin and full CACHE-002 rerun are reviewed.