code: @InternalApi
    public MutateRowsRequest toBulkProto(RequestContext requestContext) {
      String tableName =
          NameUtil.formatTableName(
              requestContext.getProjectId(), requestContext.getInstanceId(), tableId);
  
      return MutateRowsRequest.newBuilder()
          .setAppProfileId(requestContext.getAppProfileId())
          .setTableName(tableName)
          .addEntries(
              Entry.newBuilder().setRowKey(key).addAllMutations(mutation.getMutations()).build())
          .build();
    }
docs: Creates a single entry bulk {@link com.google.bigtable.v2.MutateRowsRequest}, which will be
  merged by the batching logic in the callable chain.