Don't create observationCoroutineGrpcServerInterceptor bean when micrometer context-propagation isn't present on classpath

[genuss]

Answering a bit late on the question in #282

As it was mentioned in #283 the issue is caused by context-propagation missing on a classpath. The root-cause is a bit deeper. If observation is configured spring-grpc will create a ObservationCoroutineContextServerInterceptor bean specifically for kotlin coroutine-based servers. Although this class doesn't require context-propagation directly, its dependency KotlinObservationContextElement does. That's why the kotlin-sample server starts, but then it fails to process a request silently. Enabling the logs gives the root-cause:

2025-11-10T16:29:19.911+01:00 ERROR 27640 --- [grpc-server-kotlin] [ault-executor-0] eptionHandlerInterceptor$FallbackHandler : Unknown exception

java.lang.NoClassDefFoundError: io/micrometer/context/ContextRegistry
	at io.micrometer.core.instrument.kotlin.AsContextElementKt.asContextElement(AsContextElement.kt:31) ~[micrometer-core-1.16.0-RC1.jar:1.16.0-RC1]
	at io.micrometer.core.instrument.kotlin.ObservationCoroutineContextServerInterceptor.coroutineContext(ObservationCoroutineContextServerInterceptor.kt:45) ~[micrometer-core-1.16.0-RC1.jar:1.16.0-RC1]
	at io.grpc.kotlin.CoroutineContextServerInterceptor.interceptCall(CoroutineContextServerInterceptor.kt:61) ~[grpc-kotlin-stub-1.5.0.jar:na]
	at io.grpc.ServerInterceptors$InterceptCallHandler.startCall(ServerInterceptors.java:269) ~[grpc-api-1.76.0.jar:1.76.0]
	at io.micrometer.core.instrument.binder.grpc.ObservationGrpcServerInterceptor.interceptCall(ObservationGrpcServerInterceptor.java:124) ~[micrometer-core-1.16.0-RC1.jar:1.16.0-RC1]
	at io.grpc.ServerInterceptors$InterceptCallHandler.startCall(ServerInterceptors.java:269) ~[grpc-api-1.76.0.jar:1.76.0]
	at org.springframework.grpc.server.exception.GrpcExceptionHandlerInterceptor.interceptCall(GrpcExceptionHandlerInterceptor.java:74) ~[classes/:na]
	at io.grpc.ServerInterceptors$InterceptCallHandler.startCall(ServerInterceptors.java:269) ~[grpc-api-1.76.0.jar:1.76.0]
	at io.grpc.internal.ServerImpl$ServerTransportListenerImpl.startWrappedCall(ServerImpl.java:701) ~[grpc-core-1.76.0.jar:1.76.0]
	at io.grpc.internal.ServerImpl$ServerTransportListenerImpl.access$2200(ServerImpl.java:408) ~[grpc-core-1.76.0.jar:1.76.0]
	at io.grpc.internal.ServerImpl$ServerTransportListenerImpl$1HandleServerCall.runInternal(ServerImpl.java:613) ~[grpc-core-1.76.0.jar:1.76.0]
	at io.grpc.internal.ServerImpl$ServerTransportListenerImpl$1HandleServerCall.runInContext(ServerImpl.java:603) ~[grpc-core-1.76.0.jar:1.76.0]
	at io.grpc.internal.ContextRunnable.run(ContextRunnable.java:37) ~[grpc-core-1.76.0.jar:1.76.0]
	at io.grpc.internal.SerializingExecutor.run(SerializingExecutor.java:133) ~[grpc-core-1.76.0.jar:1.76.0]
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144) ~[na:na]
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642) ~[na:na]
	at java.base/java.lang.Thread.run(Thread.java:1583) ~[na:na]
Caused by: java.lang.ClassNotFoundException: io.micrometer.context.ContextRegistry
	... 17 common frames omitted

In RC1 spring-boot-starter-actuator provided context-propagation via spring-boot-micrometer-tracing, but it's not the case anymore, see spring-projects/spring-boot#47785.

Given all that, I propose a safe solution: don't create a ObservationCoroutineContextServerInterceptor bean if context-propagation isn't available.