code: public BulkMutation add(@Nonnull ByteString rowKey, @Nonnull Mutation mutation) {
      Preconditions.checkNotNull(rowKey);
      Preconditions.checkNotNull(mutation);
  
      builder.addEntries(
          MutateRowsRequest.Entry.newBuilder()
              .setRowKey(rowKey)
              .addAllMutations(mutation.getMutations())
              .build());
      return this;
    }
docs: Add mutation for a particular row. The changes in the mutation will be applied atomic. However
  there is no guarantees about the relative ordering between mutations affecting different rows.