allow custom ObservationGrpcClientInterceptor via @ConditionalOnMissingBean

Closes: gh-246
Signed-off-by: Andrey Litvitski <andrey1010102008@gmail.com>

Author: therepanic

spring-grpc-spring-boot-autoconfigure/src/main/java/org/springframework/grpc/autoconfigure/client/GrpcClientObservationAutoConfiguration.java

@@ -18,6 +18,7 @@
 import org.springframework.boot.autoconfigure.AutoConfiguration;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.context.annotation.Bean;
 import org.springframework.grpc.client.GlobalClientInterceptor;
@@ -36,6 +37,7 @@ public class GrpcClientObservationAutoConfiguration {
 
 	@Bean
 	@GlobalClientInterceptor
+	@ConditionalOnMissingBean
 	ObservationGrpcClientInterceptor observationGrpcClientInterceptor(ObservationRegistry observationRegistry) {
 		return new ObservationGrpcClientInterceptor(observationRegistry);
 	}