internal: collect extended on json export

After this patch, the only metrics method that plugin export uses
is metrics.collect{invoke_callbacks = true, extended_format = true}.
Process API is also separated, so now a developer can reuse export
handles if he already has an output (for example, processing historical
data in Flight Recorder).

Older version was preserved for test purposes to ensure that output is
the same in new implementation.

Part of tarantool/tarantool#7725
Part of tarantool/tarantool#7728

metrics/plugins/json.lua

@@ -1,4 +1,5 @@
 local metrics = require('metrics')
+local string_utils = require('metrics.string_utils')
 local json = require('json')
 local json_exporter = {}
 
@@ -38,7 +39,8 @@ local function format_observation(obs)
     return part
 end
 
-function json_exporter.export()
+json_exporter.internal = {}
+function json_exporter.internal.collect_and_serialize_v1()
     metrics.invoke_callbacks()
     local stat = {}
 
@@ -51,4 +53,30 @@ function json_exporter.export()
     return json.encode(stat)
 end
 
+function json_exporter.format_output(output)
+    local result = {}
+    for _, coll_obs in pairs(output) do
+        for group_name, obs_group in pairs(coll_obs.observations) do
+            local metric_name = string_utils.build_name(coll_obs.name, group_name)
+            for _, obs in pairs(obs_group) do
+                table.insert(result, {
+                    metric_name = metric_name,
+                    label_pairs = format_label_pairs(obs.label_pairs),
+                    timestamp = coll_obs.timestamp,
+                    value = format_value(obs.value),
+                })
+            end
+        end
+    end
+
+    return json.encode(result)
+end
+
+function json_exporter.internal.collect_and_serialize_v2()
+    local output = metrics.collect{invoke_callbacks = true, extended_format = true}
+    return json_exporter.format_output(output)
+end
+
+json_exporter.export = json_exporter.internal.collect_and_serialize_v2
+
 return json_exporter

test/plugins/json_test.lua

@@ -78,3 +78,27 @@ g.test_number64_ull_value_parses_to_json_number = function()
     t.assert_not_equals(type(obs_ull.value), 'string', 'number64 is not casted to string on export')
     t.assert_equals(obs_ull.value, 9007199254740992ULL, 'number64 ULL parsed to corrent number value')
 end
+
+g.test_collect_and_serialize_preserves_format = function()
+    -- Prepare some data for all collector types.
+    metrics.cfg{include = 'all', exclude = {}, labels = {alias = 'router-3'}}
+
+    local c = metrics.counter('cnt', nil, {my_useful_info = 'here'})
+    c:inc(3, {mylabel = 'myvalue1'})
+    c:inc(2, {mylabel = 'myvalue2'})
+
+    c = metrics.gauge('gauge', nil, {my_useful_info = 'here'})
+    c:set(3, {mylabel = 'myvalue1'})
+    c:set(2, {mylabel = 'myvalue2'})
+
+    c = metrics.histogram('histogram', nil, {2, 4}, {my_useful_info = 'here'})
+    c:observe(3, {mylabel = 'myvalue1'})
+    c:observe(2, {mylabel = 'myvalue2'})
+
+    local output_v1 = json.decode(json_exporter.internal.collect_and_serialize_v1())
+    local output_v2 = json.decode(json_exporter.internal.collect_and_serialize_v2())
+
+    for _, obs in ipairs(output_v1) do
+        utils.find_obs(obs.metric_name, obs.label_pairs, output_v2)
+    end
+end