Describe the bug
toDouble() can potentially throw a null pointer exception, as it's only checking for the metric to exist.

Unfortunately, Kafka 3.9.1 returns a metric for addOldestOpenIteratorGauge that can have null as a metric value, leading to a NPE in our code when trying to read the metric. As the gauge exists but has a null value.

java.lang.NullPointerException: Cannot invoke "java.lang.Number.doubleValue()" because the return value of "org.apache.kafka.common.Metric.metricValue()" is null
at kafka.KafkaMetrics.toDouble(KafkaMetrics.java:326) ~[micrometer-core-1.14.7-jakartaee.jar:1.14.7-migrated-1.0.8]
at kafka.KafkaMetrics.lambda$toMetricValue$6(KafkaMetrics.java:322) ~[micrometer-core-1.14.7-jakartaee.jar:1.14.7-migrated-1.0.8]

Environment

• Micrometer version: 1.14.7
• Micrometer registry: PushMeterRegistry
• OS: Ubuntu
• Java version: 17

To Reproduce
https://github.com/ArneKlein/micrometer-npe-example will create the metric, but I couldn't get a simple enough example for reproducing the issue with a push meter registry yet

Expected behavior
toDouble returns NaN instead of null for missing metric values