Document LuaJIT getmetrics C and Lua API (#2280)

Fixes #1597 

Written by Peter Gulutzan, reviewed by Igor Munkin and Sergey Kaplun

Translated by ainoneko, translation reviewed by Patience Daur

Co-authored-by: Peter Gulutzan <pgulutzan@ocelot.ca>
Co-authored-by: Igor Munkin <imun@tarantool.org>
Co-authored-by: ainoneko <ainoneko@gmail.com>
Co-authored-by: Patience Daur <patiencedaur@gmail.com>

Author: 4 people

doc/book/app_server/index.rst

@@ -19,5 +19,6 @@ This chapter contains the following sections:
    contributing_module
    reloading_module
    luajit_memprof
+   luajit_getmetrics
    using_ide
    cookbook