Expose Athenz client metrics in a simple way

[ikhoon]

Although it is possible to expose the metrics of Athenz clients internally used in ZtsBaseClient through configureClientFactory and configureWebClient, the process is not straightforward.

ZtsBaseClient
    .builder("...")
    .configureClientFactory(builder -> builder.meterRegistry(meterRegistry))
    .configureWebClient(builder -> builder.decorator(MetricCollectingClient.newDecorator(MeterIdPrefixFunction.ofDefault("xxx")))

I propose adding a new API to configure metrics more declaratively.

ZtsBaseClient
    .builder("...")
    .enableMetrics(meterRegistry, MeterIdPrefixFunction.ofDefault(...))