Provide a simple way to disable WebClient instrumentation

[asibross]

I am looking to disable the TraceWebClientBeanPostProcessor since I have my own simple brave instrumentation for WebClient.

I cannot just exclude it because it is being created in a nested configuration in TraceWebClientAutoConfiguration and the TraceWebClientAutoConfiguration is also in charge of providing other instrumentations (RestTemplate) that I don't want to exclude.

I had to implement a BeanDefinitionRegistryPostProcessor to remove it, but would have preferred to set a property, or add the AutoConfiguration to the exclude list.

I think adding a @ConditionalOnProperty on the nested configuration will suffice.

[marcingrzejszczak]

Are you willing to add a pull request to the master branch? We can make it @ConditionalOnProperty indeed.

[marcingrzejszczak]

Fixed via #1723