If <tracerConfig> is absent, TracerConfigurator will try to pick up the Tracer instance that was registered in io.opentracing.util.GlobalTracer.
By doing this some backends like DataDog are supported out of the box since
datadog-java-agent uses Javaagent to register a Tracer in
Configuring Sample Rate
By default only 0.1% of requests are sampled, this ensures that tracing activities do not affect performance of a node.
The rate can be changed without needing to restart Solr nodes by setting the new sample rate as a cluster property using the Collections API’s CLUSTERPROP command.
For example, the following sets the samplePercentage property to 100%: