public Builder mergeStartTime(com.google.protobuf.Timestamp value) {
      if (startTimeBuilder_ == null) {
        if (((bitField0_ & 0x00000010) != 0)
            && startTime_ != null
            && startTime_ != com.google.protobuf.Timestamp.getDefaultInstance()) {
          getStartTimeBuilder().mergeFrom(value);
        } else {
          startTime_ = value;
        }
      } else {
        startTimeBuilder_.mergeFrom(value);
      }
      if (startTime_ != null) {
        bitField0_ |= 0x00000010;
        onChanged();
      }
      return this;
    }
<pre>
Required. The start time of the span. On the client side, this is the time
kept by the local machine where the span execution starts. On the server
side, this is the time when the server's application handler starts
running.
</pre>

<code>.google.protobuf.Timestamp start_time = 5 [(.google.api.field_behavior) = REQUIRED];
</code>
java-trace/proto-google-cloud-trace-v2/src/main/java/com/google/devtools/cloudtrace/v2/Span.java