feat: add getClusterMetricsAsJSON to cluster registries (#470)

[amasen02]

Summary

• Adds \getClusterMetricsAsJSON(aggregator?: string)\ to \AggregatorRegistry\ and \ClusterRegistry\ to expose aggregated cluster metrics as parsed JSON objects, mirroring \Registry#getMetricsAsJSON().
• Refactors #gather()\ and collection coordination into #aggregateMetrics(transform)\ so that both \clusterMetrics()\ and \getClusterMetricsAsJSON()\ share identical collection and worker lifecycle logic without duplicating code.
• Ensures \shutdown()\ continues to cleanly track and await in-flight aggregated metrics requests through completion before exiting.
• Updates TypeScript definitions in \index.d.ts\ for both \ClusterRegistry\ and \AggregatorRegistry.
• Adds comprehensive unit test coverage in \ est/clusterTest.js\ verifying empty cluster state, multi-worker JSON aggregation ordering, and aggregator filtering (\sum/\omit), as well as compile-time type verification in \ est/typescript.ts.
• Updates \README.md\ documentation with usage example.

Closes #470.

Verification

• 
  pm run lint\ (clean, 0 warnings/errors)
• 
  pm run check-prettier\ (all files formatted)
• 
  pm run compile-typescript\ (clean compilation)
• 
  pm run test-unit -- --coverage\ (31 suites passed, 603 tests passed, 34 snapshots passed)