[Config] Document the `Database statistics` section

Product: Tarantool
Audience/target: administrators
Root document: https://docs.d.tarantool.io/en/doc/gh-4012-memtx-config/book/admin/
SME: @ Gumix

Details

https://www.tarantool.io/en/doc/latest/release/3.0.0/#database-statistics

Related issue: https://github.com/tarantool/doc/issues/3841

Draft

Check the database statistics
-----------------------------

Tarantool provides the statistics about memory consumption for the given space or specific tuples.
Available statistics:

*   The amount of memory used for the :ref:`data of the specified space <configuration_memtx-statistics-space>`.
*   The amount of :ref:`additional memory <configuration_memtx-statistics-additional>` used for the supplementary information.
*   The :ref:`total memory usage <configuration_memtx-statistics-total>`.

Space
~~~~~

To get the amount of memory in bytes occupied by the specified space, use the :ref:`space_object:bsize() <box_space-bsize>` method.
The function returns the total number of bytes in all tuples.

..  code-block:: console

    memtx:instance001> box.space.books:bsize()
    ---
    - 70348673
    ...

Additional memory
~~~~~~~~~~~~~~~~~

To check the usage of the additional memory, use the ``space_object:stat()`` method.
The following information is provided:

-   ``header_size`` and ``field_map_size``: the size of service information.
-   ``data_size``: the actual size of data, which equals to ``space_object:bsize()``.
-   ``waste_size``: the size of memory wasted due to internal fragmentation in the `slab allocator <https://github.com/tarantool/small>`_.

.. code-block:: console

    memtx:instance001> box.space.books:stat()
    ---
    - tuple:
        memtx:
          waste_size: 1744011
          data_size: 70348673
          header_size: 2154132
          field_map_size: 0
        malloc:
          waste_size: 0
          data_size: 0
          header_size: 0
          field_map_size: 0
    ...

To get the usage of the additional memory (5 Mb) for the specified tuple, use ``tuple_object:info()``:

.. code-block:: console

    memtx:instance001> box.space.books:get('1853260622'):info()
    ---
    - data_size: 277
      waste_size: 9
      arena: memtx
      field_map_size: 0
      header_size: 10
    ...

Total memory usage
~~~~~~~~~~~~~~~~~~

To get the total memory usage in bytes for the slab allocator, use the :ref:`box.slab.info() <box_slab_info>` function:

..  code-block:: console

    memtx:instance001> box.slab.info().items_used
    ---
    - 75302024
    ...

Open the linked root document for the administrator guide and compare its existing configuration coverage with the Database statistics release note. Incorporate the supplied draft and examples so the section explains space, additional, tuple, and total memory statistics; done when administrators can follow the documented commands.