public void addAnnotation(Annotation<?> annotation) {
        if (this.annotations == null) {
            this.annotations = new ArrayList<>();
        }
        this.annotations.add(annotation);
    }
Span represent RPC

@author netspider
@author emeroad
agent-module/profiler/src/main/java/com/navercorp/pinpoint/profiler/context/SpanEvent.java