Introduce XdsLoadBalancerLifecycleObserver

[jrhee17]

Motivation:

In order to provide better telemetry into XdsLoadBalancer, this PR introduces XdsLoadBalancerLifecycleObserver.
These set of metrics are conceptually equivalent to envoy's *membership* stats.
ref: https://www.envoyproxy.io/docs/envoy/latest/configuration/upstream/cluster_manager/cluster_stats#config-cluster-manager-cluster-stats

In order to expose the internal state of XdsLoadBalancer, a LoadBalancerState interface is introduced which contains information on:

• the lifecycle of updating the loadbalancer
• endpoints per priority/locality
• panic state
• subset information

Modifications:

• HostSet has been extracted to an interface, and the default implementation has been renamed to DefaultHostSet
• residualPercentages are calculated for zone-aware load balancing to provide more meaningful insights into the internal state

Result:

• XdsLoadBalancerLifecycleObserver can be used to observe the internal state of load balancers.

[github-actions]

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build-ubicloud-standard-16-jdk-8	✅	https://ge.armeria.dev/s/cbgqqnjpfwyfi
build-ubicloud-standard-16-jdk-21-snapshot-blockhound	✅	https://ge.armeria.dev/s/vg43hm4cpvuk4
build-ubicloud-standard-16-jdk-17-min-java-17-coverage	✅	https://ge.armeria.dev/s/pmwwidi64ymna
build-ubicloud-standard-16-jdk-17-min-java-11	✅	https://ge.armeria.dev/s/brqnmtjsrbhik
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