feat(ts): add observability metrics

[lan17]

Summary

• add observability hooks for requests, misses, disabled cache paths, cache/fallback errors, invalidation calls, get/fallback/serialization timers, and serialized value sizes
• add Prometheus support via prom-client with custom registry/prefix support and duplicate collector reuse
• support custom metrics adapters for non-Prometheus integrations
• improve cache-vs-fallback error classification and disabled-reason labels
• document Milestone 4 metrics behavior and add Given/When/Then behavioral coverage

Validation

• pnpm install --frozen-lockfile
• pnpm ts:gcache:typecheck
• pnpm ts:gcache:test — 42 tests, coverage: 97.3% statements / 93.4% branches / 97.8% functions / 97.55% lines
• pnpm ts:gcache:build

Stack

Stacked on #135 / lev/ts-m3-config-ramp. This PR intentionally leaves targeted invalidation/watermarks and framework integrations for later milestone PRs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.47%. Comparing base (41a08a5) to head (d126ede).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           lev/ts-m3-config-ramp     #136   +/-   ##
======================================================
  Coverage                  97.47%   97.47%           
======================================================
  Files                         13       13           
  Lines                        673      673           
======================================================
  Hits                         656      656           
  Misses                        17       17           

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[lan17]

Closing this stacked milestone PR in favor of the consolidated peak PR #137.