Add OpenTelemetry metrics to Commons Pool 2 XA implementation

[Copilot]

Exposes comprehensive pool metrics from the Apache Commons Pool 2 XA connection pool to OpenTelemetry, matching HikariCP's observability capabilities.

Metrics Exposed

Gauges: active/idle/pending connections, max/min pool size, utilization %, created/destroyed counts
Counters: pool exhaustion events, validation failures, leak detections, acquisition time/count

All metrics tagged with pool.name attribute for multi-pool environments.

Implementation

• PoolMetrics interface - Abstraction layer for metrics collection
• OpenTelemetryPoolMetrics - OpenTelemetry integration with 13 metrics via Meter API
• NoOpPoolMetrics - Zero-overhead implementation when metrics disabled
• PoolMetricsFactory - Automatic detection and instantiation based on OpenTelemetry availability
• MetricsAwareHousekeepingListener - Wrapper to capture housekeeping events (leak detection) as metrics

Metrics update on pool operations (borrow/return/invalidate) and integrate with existing Prometheus exporter in ojp-server.

Configuration

config.put("xa.metrics.enabled", "true");      // Optional, default: auto-detect
config.put("xa.poolName", "orders-xa-pool");   // Optional, default: "ojp-xa-pool"

Design Notes

• OpenTelemetry API is provided scope - no hard dependency
• Automatic activation when OpenTelemetry available on classpath
• NoOp implementation ensures zero overhead when disabled
• Interface-based design allows future extension to other frameworks (Micrometer, etc.)

Example Prometheus output:

ojp_xa_pool_connections_active{pool_name="ojp-xa-pool"} 5.0
ojp_xa_pool_connections_idle{pool_name="ojp-xa-pool"} 15.0
ojp_xa_pool_connections_utilization{pool_name="ojp-xa-pool"} 25.0

See METRICS.md for complete reference, monitoring best practices, and alerting recommendations.

Original prompt

Do an analysis on exposing metrics from the OJP XA Apache Commons Pool 2 implementation to Open Telemetry, similarly to what HikariCP does.

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