code: @Deprecated
      public DBObject group(final GroupCommand cmd, final ReadPreference readPreference) {
          return toDBList(executor.execute(cmd.toOperation(getNamespace(), getDefaultDBObjectCodec(), retryReads), readPreference,
                  getReadConcern()));
      }
docs: Group documents in a collection by the specified key and performs simple aggregation functions such as computing counts and sums.
  This is analogous to a {@code SELECT ... GROUP BY} statement in SQL.
  
  @param cmd            the group command
  @param readPreference {@link ReadPreference} to be used for this operation
  @return a document with the grouped records as well as the command meta-data
  @mongodb.driver.manual reference/command/group/ Group Command
  @deprecated The group command was deprecated in MongoDB 3.4