New `box.runtime.info().tuple` metric

Related dev. issue(s): https://github.com/tarantool/tarantool/commit/b56e012bc9f827b8cecdcf78ec3f8bf0860a335b.

Product: Tarantool
Since: 2.10.0-rc1
Root document: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_slab/runtime_info/
SME: @ Totktonada

Details

It allows to track memory allocated for tuples on runtime arena. It does
not count tuples owned by memtx and vinyl, but tracks so called runtime
tuples. The most common example is a tuple created by the
box.tuple.new(<...>) function.

Example:

tarantool> box.runtime.info().tuple -- 0
tarantool> box.tuple.new({})
tarantool> box.runtime.info().tuple -- 160
tarantool> box.tuple.new({})
tarantool> box.runtime.info().tuple -- 320
tarantool> collectgarbage()
tarantool> box.runtime.info().tuple -- 160

Definition of done

• You have agreed with @Totktonada on the best way to describe the parameter.
• The parameter is added in the documentation.

Start with the root document for box.slab.runtime_info and review the existing description of box.runtime.info().tuple. Confirm the parameter wording with @Totktonada, then add the tuple metric documentation and verify that the example and definition-of-done requirements are covered.