Safely destructure numerical values when configuring Kafka Metrics

[Hc747]

Related to:

• ClassCastException (String object cannot be cast to a Number object) in Micrometer while starting up Micronaut 3.8.0 app with Kafka metrics  #671
• Skip string metrics (like these: 'kafka.producer.commit-id', 'version', 'commitId', 'applicationId', 'topologyDescription') #668

Addresses the following:

05:06:46.307 [elastic-apm-shared] WARN  io.micrometer.core.instrument.internal.DefaultGauge - Failed to apply the value function for the gauge 'kafka.consumer.commit-id'. Note that subsequent logs will be logged at debug level.
java.lang.ClassCastException: class java.lang.String cannot be cast to class java.lang.Number (java.lang.String and java.lang.Number are in module java.base of loader 'bootstrap')
	at io.micronaut.configuration.kafka.metrics.builder.KafkaMetricMeterTypeBuilder.lambda$build$0(KafkaMetricMeterTypeBuilder.java:137) ~[XXX.jar:?]
	at io.micrometer.core.instrument.Gauge.lambda$builder$0(Gauge.java:58) ~[XXX.jar:?]
	at io.micrometer.core.instrument.StrongReferenceGaugeFunction.applyAsDouble(StrongReferenceGaugeFunction.java:48) ~[XXX.jar:?]
	at io.micrometer.core.instrument.internal.DefaultGauge.value(DefaultGauge.java:53) ~[invoice.jar:?]
	at co.elastic.apm.agent.micrometer.MicrometerMeterRegistrySerializer.serializeMetricSet(MicrometerMeterRegistrySerializer.java:137) ~[?:?]
	at co.elastic.apm.agent.micrometer.MicrometerMeterRegistrySerializer.serialize(MicrometerMeterRegistrySerializer.java:90) ~[?:?]
	at co.elastic.apm.agent.micrometer.MicrometerMetricsReporter.run(MicrometerMetricsReporter.java:192) ~[?:?]
	at co.elastic.apm.agent.micrometer.MicrometerMetricsReporter.run(MicrometerMetricsReporter.java:145) ~[?:?]
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:577) ~[?:?]
	at java.util.concurrent.FutureTask.runAndReset(FutureTask.java:358) ~[?:?]
	at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:305) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[?:?]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[?:?]
	at co.elastic.apm.agent.util.ExecutorUtils$2.run(ExecutorUtils.java:99) ~[?:?]