ADP-1490: add new Prometheus client for Flink scrape compatibility and wire the existing collector to that

[varkey98]

Summary

• Adding a new prometheus registry for serving both flink related metrics and the metrics which get initiated via PlatformMetricsRegistry.
• Add SharedMeterRegistry.getOrCreatePrometheusRegistry(serviceName, port) for Flink TaskManagers: one-time process init via a new config-free PlatformMetricsRegistry.initMetricsRegistryForFlink(String) (pins the prometheus reporter and falls back to defaults for everything else), binds a standalone Prometheus scrape endpoint using HTTPServer (prometheus-metrics-exporter-httpserver), and returns the shared PrometheusRegistry so a Flink reporter can bridge Flink-native metrics onto the same endpoint. Application meters still go through PlatformMetricsRegistry.getMeterRegistry(). Existing initMetricsRegistry(String, Config) callers keep API parity.
• Expose the shared registry via PlatformMetricsRegistry.getPrometheusRegistry().

Test plan

• :platform-metrics:test and :platform-service-framework:test pass
• Confirm /metrics scrapes Micrometer + Dropwizard metrics after service startup
• Confirm the deprecated simpleclient CollectorRegistry metrics are bridged onto the shared PrometheusRegistry
• Confirm Flink path can initialize via SharedMeterRegistry.getOrCreatePrometheusRegistry(serviceName, port) without loading typesafe-config as parent-first
• Confirm existing initMetricsRegistry(serviceName, config) callers still behave as before

[github-actions]

Test Results

32 tests  ±0   32 ✅ ±0   18s ⏱️ -1s
 9 suites ±0    0 💤 ±0 
 9 files   ±0    0 ❌ ±0 

Results for commit 66589cc. ± Comparison against base commit 4db4b1d.

♻️ This comment has been updated with latest results.