private com.google.protobuf.SingleFieldBuilderV3<
                com.google.devtools.cloudtrace.v2.Span.Attributes,
                com.google.devtools.cloudtrace.v2.Span.Attributes.Builder,
                com.google.devtools.cloudtrace.v2.Span.AttributesOrBuilder>
            getAttributesFieldBuilder() {
          if (attributesBuilder_ == null) {
            attributesBuilder_ =
                new com.google.protobuf.SingleFieldBuilderV3<
                    com.google.devtools.cloudtrace.v2.Span.Attributes,
                    com.google.devtools.cloudtrace.v2.Span.Attributes.Builder,
                    com.google.devtools.cloudtrace.v2.Span.AttributesOrBuilder>(
                    getAttributes(), getParentForChildren(), isClean());
            attributes_ = null;
          }
          return attributesBuilder_;
        }
<pre>
A set of attributes on the annotation. You can have up to 4 attributes
per Annotation.
</pre>

<code>.google.devtools.cloudtrace.v2.Span.Attributes attributes = 2;</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Span.java