api: allow to collect only default metrics

After this patch, it is possible to collect only default metrics in Lua.
Collector metainfo is used to decide which metric is default and which
is not. Beware that collect does not control the list of enabled metrics
-- if some (or all) default metrics were disabled or weren't enabled,
they won't be appear in observation.

The motivation is a requirement from Tarantool Feedback Daemon [1] to
collect default metrics, but not the user ones.

1. tarantool/tarantool#8192

Part of tarantool/tarantool#8192, part of tarantool/tarantool#7725

Author: DifferentialOrange

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ability to set metainfo for collectors
 - Set `metainfo.default` to `true` for all collectors
   from `enable_default_metrics()` and psutils collectors
+- `default_only` option for `metrics.collect()`
 
 ### Fixed
 

doc/monitoring/api_reference.rst

@@ -360,6 +360,7 @@ Metrics functions
     :param table opts: table of collect options:
 
       * ``invoke_callbacks`` -- if ``true``, ``invoke_callbacks()`` is triggerred before actual collect.
+      * ``default_only`` -- if ``true``, observations contain only default metrics (``metainfo.default = true``).
 
 ..  class:: registry
 

metrics/init.lua

@@ -41,15 +41,32 @@ local function invoke_callbacks()
     return registry:invoke_callbacks()
 end
 
+local function get_collector_values(collector, result)
+    for _, obs in ipairs(collector:collect()) do
+        table.insert(result, obs)
+    end
+end
+
 local function collect(opts)
-    checks({invoke_callbacks = '?boolean'})
+    checks({invoke_callbacks = '?boolean', default_only = '?boolean'})
     opts = opts or {}
 
     if opts.invoke_callbacks then
         registry:invoke_callbacks()
     end
 
-    return registry:collect()
+    local result = {}
+    for _, collector in pairs(registry.collectors) do
+        if opts.default_only then
+            if collector.metainfo.default then
+                get_collector_values(collector, result)
+            end
+        else
+            get_collector_values(collector, result)
+        end
+    end
+
+    return result
 end
 
 local function clear()

test/metrics_test.lua

@@ -199,3 +199,38 @@ g.test_default_metrics_metainfo = function()
             ('default collector %s has metainfo label "default"'):format(k))
     end
 end
+
+local collect_default_only_cases = {
+    default = {
+        args = {invoke_callbacks = true},
+        custom_expected = true,
+    },
+    ['true'] = {
+        args = {invoke_callbacks = true, default_only = true},
+        custom_expected = false,
+    },
+    ['false'] = {
+        args = {invoke_callbacks = true, default_only = false},
+        custom_expected = true,
+    },
+}
+
+for name, case in pairs(collect_default_only_cases) do
+    g['test_collect_default_only_' .. name] = function()
+        metrics.enable_default_metrics()
+        local c = metrics.gauge('custom_metric')
+        c:set(42)
+
+        local observations = metrics.collect(case.args)
+
+        local default_obs = utils.find_metric('tnt_info_memory_lua', observations)
+        t.assert_not_equals(default_obs, nil)
+
+        local custom_obs = utils.find_metric('custom_metric', observations)
+        if case.custom_expected then
+            t.assert_not_equals(custom_obs, nil)
+        else
+            t.assert_equals(custom_obs, nil)
+        end
+    end
+end