code: static Set<String> annotationKeys(Span span) {
      Set<String> annotationKeys = new LinkedHashSet<>();
      String localServiceName = span.localServiceName();
      if (localServiceName == null) return Collections.emptySet();
      for (Annotation a : span.annotations()) {
        // don't index core annotations as they aren't queryable
        if (CORE_ANNOTATIONS.contains(a.value())) continue;
        annotationKeys.add(localServiceName + ":" + a.value());
      }
      for (Map.Entry<String, String> e : span.tags().entrySet()) {
        if (e.getValue().length() <= LONGEST_VALUE_TO_INDEX) {
          annotationKeys.add(localServiceName + ":" + e.getKey());
          annotationKeys.add(localServiceName + ":" + e.getKey() + ":" + e.getValue());
        }
      }
      return annotationKeys;
    }
docs: Returns keys that concatenate the serviceName associated with an annotation or tag.
  
  @see QueryRequest#annotationQuery()