Add `@ConditionalOnMissingBean` annotation in `GrpcClientObservationAutoConfiguration` class

[bloodyt3ars]

In the autoconfiguration class org.springframework.grpc.autoconfigure.client.GrpcClientObservationAutoConfiguration an ObservationGrpcClientInterceptor is created. This Interceptor is annotated with @GlobalClientInterceptor. That is, under the conditions specified on the class, this Interceptor will always be created and will be included in all channels. It has a setCustomConvention method, but to use it, you need to create a new instance and manually add сustom сonvention there.

I proposal adding a @ConditionalOnMissingBean annotation to this method so that a custom convention can be configured if needed.

An example of how I would like to use this:

    @Bean
    @GlobalClientInterceptor
    ObservationGrpcClientInterceptor observationGrpcClientInterceptor(
            ObservationGrpcClientInterceptor interceptor) {
        interceptor.setCustomConvention(new CustomGrpcClientObservationConvention());
        return interceptor;
    }

[dsyer]

The autoconfig is conditional on spring.grpc.client.observation.enabled=true so you could set that to false and provide your own @Bean of type ObservationGrpcClientInterceptor. Your suggestion for the missing bean condition also makes perfect sense (send a PR if you prefer that approach).

The pattern in you snippet above is wrong I think in any case (you can't have 2 @Bean methods that both return a ObservationGrpcClientInterceptor).