@java.lang.Override
  public void writeTo(com.google.protobuf.CodedOutputStream output) throws java.io.IOException {
    if (((bitField0_ & 0x00000001) != 0)) {
      output.writeMessage(1, getCaptureTime());
    }
    if (((bitField0_ & 0x00000002) != 0)) {
      output.writeMessage(2, getType());
    }
    if (((bitField0_ & 0x00000004) != 0)) {
      output.writeMessage(3, getMetadata());
    }
    if (((bitField0_ & 0x00000008) != 0)) {
      output.writeMessage(4, getServerMetadata());
    }
    if (((bitField0_ & 0x00000010) != 0)) {
      output.writeMessage(5, getSeriesMetadata());
    }
    if (flags_ != 0) {
      output.writeInt32(6, flags_);
    }
    if (!com.google.protobuf.GeneratedMessageV3.isStringEmpty(traceContext_)) {
      com.google.protobuf.GeneratedMessageV3.writeString(output, 7, traceContext_);
    }
    getUnknownFields().writeTo(output);
  }
<pre>
Immutable. Header string for tracing across services. It should be set when
the packet is first arrived in the stream server.

The input format is a lowercase hex string:
  - version_id: 1 byte, currently must be zero - hex encoded (2 characters)
  - trace_id: 16 bytes (opaque blob) - hex encoded (32 characters)
  - span_id: 8 bytes (opaque blob) - hex encoded (16 characters)
  - trace_options: 1 byte (LSB means tracing enabled) - hex encoded (2
  characters)
Example: "00-404142434445464748494a4b4c4d4e4f-6162636465666768-01"
          v  trace_id                         span_id          options
</pre>

<code>string trace_context = 7 [(.google.api.field_behavior) = IMMUTABLE];</code>

@return The bytes for traceContext.
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/PacketHeader.java