See the documentation request below for details.

Fixes #5872

@TarantoolBot document
Title: New `box.runtime.info().tuple` metric

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:

```lua
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
```