code: function TraceSpan(name, spanId, parentSpanId) {
    this.name = name;
    this.parentSpanId = parentSpanId;
    this.spanId = spanId;
    this.kind = 'RPC_CLIENT';
    this.labels = {};
    this.startTime = (new Date()).toISOString();
    this.endTime = '';
  }
docs: Creates a trace span object.
  @constructor