api: add selector-based filtering for custom metrics

[mandesero]

This patch adds selector-based filtering for custom metrics.

Before this patch, the metrics module allowed applications to register custom collectors and callbacks, but there was no common way to group them and enable or disable them later through metrics.cfg().

The patch introduces selectors for custom metrics. A selector may be attached to a collector or callback via metadata:

local metrics = require('metrics')

metrics.register_callback(function()
    in_flight:set(get_in_flight())
end, {selector = 'roles.crud-router'})

For convenience, metrics may also be registered through a namespace:

local metrics = require('metrics')

local crud = metrics.namespace('roles.crud-router')

crud:gauge('crud_requests'):set(1)
crud:gauge('crud_errors'):set(1)

The namespace prefixes registered collectors with hierarchical selectors:

roles.crud-router.crud_requests
roles.crud-router.crud_errors

Selectors can be filtered with the new metrics.set_filter() API:

metrics.set_filter(
    {{selector = 'roles.crud-router'}},
    {{selector = 'roles.crud-router.crud_errors'}}
)

include enables matching selectors, exclude disables matching selectors, and exclude has priority. Matching is hierarchical, so roles.crud-router matches roles.crud-router.crud_requests.

metrics.cfg() now also accepts custom selector strings in include and exclude:

metrics.cfg{
    include = {'info', 'roles.crud-router'},
    exclude = {'roles.crud-router.crud_errors'},
}

Built-in Tarantool metric groups keep the existing behavior. Unknown strings in include / exclude are treated as custom selectors and are applied through the registry filter.

This lets applications and Tarantool roles expose custom metrics while still using the existing metrics.include / metrics.exclude configuration model.

Part of tarantool/tarantool#11358

[patapenka-alexey]

Please add this test, it indicates break in backward compatibility.

group.test_cfg_clean_user_metrics = function(cg)
    cg.server:exec(function()
        local metrics = require('metrics')

        -- Register a custom metric WITHOUT a selector (how all apps did it before)
        local my_gauge = metrics.gauge('my_custom_metric', 'My custom metric')
        my_gauge:set(42)

        metrics.cfg{
            include = {'info'}, -- before the patch, it turn off only default metrics, but now it hides custom metrics
            exclude = {},
        }

        local observations = metrics.collect{invoke_callbacks = true}

        local found = false
        for _, obs in ipairs(observations) do
            if obs.metric_name == 'my_custom_metric' then
                found = true
                break
            end
        end

        t.assert_equals(found, true)
    end)
end

[bigbes]

Ty for you patch! Logic seems legit, but it's new functionality test coverage worth to be extended (for example - more cases for 'set_filter', reconfiguration, etc)

[patapenka-alexey]

Thank you for the patch!