docs: @VisibleForTesting
    static Timestamp convertTimestamp(io.opencensus.common.Timestamp censusTimestamp) {
      if (censusTimestamp.getSeconds() < 0) {
        // StackDriver doesn't handle negative timestamps.
        return Timestamp.newBuilder().build();
      }
      return Timestamp.newBuilder()
          .setSeconds(censusTimestamp.getSeconds())
          .setNanos(censusTimestamp.getNanos())
          .build();
    }