NPE occurs when AOP is applied to a method that returns CompletableFuture

[laststem]

Describe the bug
Returning null from a method that returns CompletableFuture will result in an NPE in AOP.

@Timed
fun doSomething(): CompletableFuture<Int>? {
    return if (condition) {
        CompletableFuture.completedFuture(1)
    } else { 
        null
    }
}

Because AOP considers it to be non-null if it is of type CompletionStage. (code)

As a result, the original method did not cause an NPE, but the AOP caused an NPE, so the calling thread was affected.

If you agree, please let me know if I can create a PR.

Environment

• Micrometer version 1.14.1
• OS: macOS
• Java version: 17

To Reproduce

@SpringBootApplication
class SpringPlaygroundApplication(
    private val testComponent: TestComponent
) : ApplicationRunner {
    override fun run(args: ApplicationArguments?) {
        val result = testComponent.test()
        println("result: $result") // No output is produced as NPE is thrown.
        Thread.sleep(10000)
    }
}

@Configuration
class TestCfg(
    private val meterRegistry: MeterRegistry,
) {
    @Bean
    fun timedAop() = TimedAspect(meterRegistry)
}

fun main(args: Array<String>) {
    runApplication<SpringPlaygroundApplication>(*args)
}

@Component
class TestComponent {
    @Timed
    fun test(): CompletableFuture<Int>? {
         return null
    }
}

Expected behavior
AOP needs to be aware that values ​​of type CompletionStage can be null.

Also, exceptions raised in AOP should not affect the original method.

Additional context
Add any other context about the problem here, e.g. related issues.

[shakuzen]

Thanks for the detailed report. Indeed it sounds like a bug. We'd be happy to review a pull request for a fix. It looks like CountedAspect and ObservedAspect also suffer from the same bug.