Per-module logging

[andreyaksenov]

• Added the description for the box.cfg.log_modules configuration property.
• Added the log.new() function description to the log module API reference.
• Added a note that environment variables are taken into account if log.cfg() is called before box.cfg().

Note that doc code snippets are included from testable examples placed in the doc/code_snippets folder for making sure they are correct and runnable.

[Gumix]

The following points from #3264 are missed:

It is possible to set custom log levels for files (modules) that just use default logger without creating a new instance by log.new()

The name of a module is determined automatically during the execution of require('log') in the module's source code. The name is derived from its filename, including a part of the path.

Example: https://github.com/tarantool/tarantool/blob/fa6ae4a3c366f7fec3fd57403bca313cf762904e/test/box-luatest/gh_3211_module/testmod.lua

https://github.com/tarantool/tarantool/blob/fa6ae4a3c366f7fec3fd57403bca313cf762904e/test/box-luatest/gh_3211_per_module_log_level_test.lua#L175-L179

There is a special module name "tarantool", which allows to set log level for Tarantool core messages. To be more precise, it sets log level for all messages logged from the non-Lua code, including user C modules.

Example: https://github.com/tarantool/tarantool/blob/fa6ae4a3c366f7fec3fd57403bca313cf762904e/test/box-luatest/gh_3211_per_module_log_level_test.lua#L190-L194

[andreyaksenov]

Thanks! Updated the docs as follows to reflect missed points:

• Updated the box.cfg.log_modules description to show that different module types are supported.
• Added 3 examples for all module types (file, custom, and tarantool).
• Added the modules property to the log.cfg({}) description.

[xuniq]

Suggested change

	such as ``TT_LOG``, ``TT_LOG_LEVEL``, and so on.
	such as ``TT_LOG`` and ``TT_LOG_LEVEL``.

[xuniq]

Suggested change

	To create the ``module1`` and ``module1`` modules, call the ``new()`` function:
	To create the ``module1`` and ``module2`` loggers, call the ``new()`` function: