Disable tracerresolver configuration

Currently, if I want to use tracerresolver there is no way how to tell auto-config that I would like to use NoopTracer to quickly “disable the instrumentation”. Probably we should provide a configuration property which would use Noop or resolve the tracer.

cc @objectiser @jpkrohling

[pavolloffay]

To clarify this issue, the use case is to provide a way to quickly disable instrumentation.

I can think of two solutions:

1. disable all auto-configs with a property

This is by means is the best from the performance perspective. However, if users are using tracer inside the app it can fail in some scenarios (e.g. fails to autowire tracer bean).

2. use noop tracer

The only issue I currently have with noop tracer is that it does not use span source inside so switching from real tracer to noop can result in NPEs (e.g. tracer.activeSpan() retruns null (https://github.com/opentracing/opentracing-java/blob/master/opentracing-noop/src/main/java/io/opentracing/noop/NoopTracer.java#L41)). I would like to address this in the next version of the API.

When somebody is not using tracerresolver and would like to disable instrumentation it can simply return NoopTracer bean or use:

@ConditionalOnProperty(name = "instrumenation.enabled", havingValue = "true", matchIfMissing = true)
@Bean
Tracer tracer() {...}

So when he start app using java -jar -Dinstrumentation.enabled=false foo.jar our auto-config will use NoopTracer.

However, if you are using tracerrresolver you cannot do ^^^. Tracer impl can be removed from classpath, however, it is not ideal for k8s deployment where you would like to control things via env properties.

[pavolloffay]

I will close this for now.

At the moment all auto-configurations can be disabled with a property. Tracer configuration has been moved here https://github.com/opentracing-contrib/java-spring-tracer-configuration.