Add OpenTelemetry metrics support for RelativeLoadBalancerStrategy

[aadityaraj7769]

Summary

This PR adds OpenTelemetry (OTel) metrics instrumentation to the RelativeLoadBalancerStrategy.

Changes

1. New Interface: Added RelativeLoadBalancerStrategyOtelMetricsProvider interface for collecting load balancer metrics via OpenTelemetry

• Per-call host latency measurements (recorded via a per-call duration listener on each TrackerClient)
• Host health tracking (total hosts, unhealthy hosts, quarantined hosts)
• Hash ring sizing

2. No-op Implementation: Added NoOpRelativeLoadBalancerStrategyOtelMetricsProvider as default implementation when metrics are disabled

3. Integration:

• Integrated metrics provider into StateUpdater constructor with dependency injection pattern
• Per-call host latency is emitted via TrackerClient.setPerCallDurationListener(), which fires on every individual request — allowing OTel to automatically compute percentiles (p50, p90, p99), averages, min, max, and standard deviation
• Gauge metrics (host counts, quarantine counts, hash ring points) are emitted after every scheduled partition state update via emitOtelMetrics()
• Added constructor overload in RelativeLoadBalancerStrategyFactory to accept a custom RelativeLoadBalancerStrategyOtelMetricsProvider
• All metrics are tagged with two dimensions: serviceName and scheme

4. Metrics Tracked:

• Histogram: per-host per-call latency
• Gauges: total hosts across all partitions, unhealthy host count, quarantine host count, total points in hash ring

Backward Compatibility

• Fully backward compatible — all existing constructors default to NoOpRelativeLoadBalancerStrategyOtelMetricsProvider

About RelativeLoadBalancerStrategySensor

The RelativeLoadBalancerStrategy Sensor tracks metrics for D2's relative load balancer, which dynamically adjusts host health scores (0.0–1.0) based on per-host call statistics relative to the overall cluster. It monitors host latency distributions, cluster health composition, and hash ring sizing. This sensor enables server-side observability for how traffic is being distributed across hosts in a service cluster.

New OTel Metrics

Metric Naming Pattern: `D2.RelativeLb.<Metric>

Dimensions

Attribute Key	Description	Applied To
D2.Service.Name	The service being load-balanced	All metrics
D2.Scheme	Load balancer scheme (e.g., http, https)	All metrics
D2.Host.Status	"Unhealthy" or "Quarantine"	DegradedHostsCount only

ExponentialHistogram

• D2.RelativeLb.HostLatency (ms) — Records each host's average latency per call. OTel automatically computes p50, p90, p99, average, min, max, and standard deviation from the distribution.

Gauges

• D2.RelativeLb.AllPartitionHostsCount ({host}) — Total number of hosts across all partitions regardless of health status; the full host population the load balancer is aware of
• D2.RelativeLb.DegradedHostsCount ({host}) — Number of hosts in a degraded state, grouped by D2.Host.Status:
  • D2.Host.Status = "Unhealthy" — hosts whose health score has been reduced due to high latency or error rate
  • D2.Host.Status = "Quarantine" — hosts currently in quarantine pending health check recovery
• D2.RelativeLb.PointsInHashRing ({point}) — Total number of points in the consistent hash ring, reflecting the effective traffic weight distribution across hosts