Bump metrics package submodule. Commits from PRs [1-4] affect
Tarantool, the other ones are related to module infrastructure.

1. https://github.com/tarantool/metrics/pull/482
2. https://github.com/tarantool/metrics/pull/483
3. https://github.com/tarantool/metrics/pull/484
4. https://github.com/tarantool/metrics/pull/491

NO_DOC=doc is a part of submodule

Rework "running tests with built-in package" assert since now checks is
a callable table package with subpackages instead of a single function.

NO_DOC=No tagged version since checks initial embedding
NO_CHANGELOG=No tagged version since checks initial embedding

tarantool/metrics [1] is a lua module (distributed as a separate rock)
for metrics aggregation and export. After this patch, it will be a part
of the tarantool binary.

1. https://github.com/tarantool/metrics

Part of #7725

@TarantoolBot document
Title: embedded metrics

Now tarantool has metrics module on its board. metrics is a lua module
previously distributed as a separate rock which is widely used by
tarantool applications. Metrics has its own documentation section:
https://www.tarantool.io/en/doc/latest/book/monitoring/ , but it doesn't
yet mention anything about the embedding.

tarantool/checks [1] is a lua module (distributed as a separate rock)
for function input validation. After this patch, it will a part of
the tarantool binary.

1. https://github.com/tarantool/checks

Closes #7726
Needed for #7725

@TarantoolBot document
Title: embedded checks

Now tarantool has checks module on its board. checks is a lua module
previously distributed as a separate rock which is widely used by
many other tarantool lua modules (like cartridge, metrics and crud) and
tarantool applications. checks has its own repo with README covering its
API usage: https://github.com/tarantool/checks/blob/master/README.md .

After adding the autorequiring luatest [1,2], there is no need to use
the following approach now:

```
local t = require('luatest')
local g = t.group()

server:exec(function()
    local t = require('luatest') -- duplicate
    t.assert(...)
end)
```

Modern approach looks like:

```
local t = require('luatest')
local g = t.group()

-- `t` already available in the remote server
server:exec(function() t.assert(...) end)

-- also it works with any variable
local my_custom_t = require('luatest')

server:exec(function()
    my_custom_t.assert(...) -- already available
end)
```

[1] tarantool/luatest#277
[2] tarantool/luatest#289

Part of tarantool/luatest#233

NO_DOC=test fix
NO_TEST=test fix
NO_CHANGELOG=test fix

tarantool.debug.getsources() used to provide
lua sources only for libserver lua modules,
and was not providing sources for src/box/lua/*
modules.

Now this code works:
```lua
local tnt = require 'tarantool'
local code1 = tnt.debug.getsources('box/session')
local code1 = tnt.debug.getsources('@builtin/box/session..lua')
```

Closes #7839

NO_DOC=bugfix
NO_CHANGELOG=later

Extend Tarantool kernel internal API with the call
`tarantool.debug.getsources()` to allow to retrieve sources
of a Tarantool `builtin/*` modules to show them in the
debugger shell.

Created simple luatest script for checking consistency
of a values returned from `require 'tarantool'.debug.getsources()`
and an ctual script file content we expected to receive.

NO_DOC=see future commit
NO_CHANGELOG=see future commit