@java.lang.Override
  public void writeTo(com.google.protobuf.CodedOutputStream output) throws java.io.IOException {
    if (spanId_ != 0L) {
      output.writeFixed64(1, spanId_);
    }
    if (kind_
        != com.google.devtools.cloudtrace.v1.TraceSpan.SpanKind.SPAN_KIND_UNSPECIFIED.getNumber()) {
      output.writeEnum(2, kind_);
    }
    if (!com.google.protobuf.GeneratedMessageV3.isStringEmpty(name_)) {
      com.google.protobuf.GeneratedMessageV3.writeString(output, 3, name_);
    }
    if (((bitField0_ & 0x00000001) != 0)) {
      output.writeMessage(4, getStartTime());
    }
    if (((bitField0_ & 0x00000002) != 0)) {
      output.writeMessage(5, getEndTime());
    }
    if (parentSpanId_ != 0L) {
      output.writeFixed64(6, parentSpanId_);
    }
    com.google.protobuf.GeneratedMessageV3.serializeStringMapTo(
        output, internalGetLabels(), LabelsDefaultEntryHolder.defaultEntry, 7);
    getUnknownFields().writeTo(output);
  }
<pre>
Collection of labels associated with the span. Label keys must be less than
128 bytes. Label values must be less than 16 kilobytes (10MB for
`/stacktrace` values).

Some predefined label keys exist, or you may create your own. When creating
your own, we recommend the following formats:

* `/category/product/key` for agents of well-known products (e.g.
  `/db/mongodb/read_size`).
* `short_host/path/key` for domain-specific keys (e.g.
  `foo.com/myproduct/bar`)

Predefined labels include:

*   `/agent`
*   `/component`
*   `/error/message`
*   `/error/name`
*   `/http/client_city`
*   `/http/client_country`
*   `/http/client_protocol`
*   `/http/client_region`
*   `/http/host`
*   `/http/method`
*   `/http/path`
*   `/http/redirected_url`
*   `/http/request/size`
*   `/http/response/size`
*   `/http/route`
*   `/http/status_code`
*   `/http/url`
*   `/http/user_agent`
*   `/pid`
*   `/stacktrace`
*   `/tid`
</pre>

<code>map&lt;string, string&gt; labels = 7;</code>
java-trace/proto-google-cloud-trace-v1/src/main/java/com/google/devtools/cloudtrace/v1/TraceSpan.java