public Builder setDescription(com.google.devtools.cloudtrace.v2.TruncatableString value) {
          if (descriptionBuilder_ == null) {
            if (value == null) {
              throw new NullPointerException();
            }
            description_ = value;
          } else {
            descriptionBuilder_.setMessage(value);
          }
          bitField0_ |= 0x00000001;
          onChanged();
          return this;
        }
<pre>
A user-supplied message describing the event. The maximum length for
the description is 256 bytes.
</pre>

<code>.google.devtools.cloudtrace.v2.TruncatableString description = 1;</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Span.java