@java.lang.Override
  public java.lang.String getTraceContext() {
    java.lang.Object ref = traceContext_;
    if (ref instanceof java.lang.String) {
      return (java.lang.String) ref;
    } else {
      com.google.protobuf.ByteString bs = (com.google.protobuf.ByteString) ref;
      java.lang.String s = bs.toStringUtf8();
      traceContext_ = s;
      return s;
    }
  }
<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 traceContext.
java-visionai/proto-google-cloud-visionai-v1/src/main/java/com/google/cloud/visionai/v1/PacketHeader.java