Allow create summary without objectives

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 - be gentle to http routes, don't leave gaps in the array
   [#246](https://github.com/tarantool/metrics/issues/246)
+- allow to create summary without observations [#265](https://github.com/tarantool/metrics/issues/265)
 
 ### Added
 - `tnt_clock_delta` metric to compute clock difference on instances

metrics/collectors/summary.lua

@@ -17,9 +17,11 @@ function Summary:new(name, help, objectives, params)
     obj.age_buckets_count = params.age_buckets_count or 1
     obj.observations = {}
 
-    obj.quantiles = {}
-    for q, _ in pairs(objectives) do
-        table.insert(obj.quantiles, q)
+    if obj.objectives then
+        obj.quantiles = {}
+        for q, _ in pairs(objectives) do
+            table.insert(obj.quantiles, q)
+        end
     end
     return obj
 end

test/collectors/summary_test.lua

@@ -163,6 +163,22 @@ g.test_summary_wrong_label = function()
         instance.observe, instance, 1, {quantile = 0.5})
 end
 
+g.test_create_summary_without_observations = function()
+    local ok, summary = pcall(metrics.summary, 'plain_summary')
+    t.assert(ok, summary)
+    summary:observe(0)
+
+    local summary_metrics = utils.find_metric('plain_summary_count', metrics.collect())
+    t.assert_equals(#summary_metrics, 1)
+
+    summary_metrics = utils.find_metric('plain_summary_sum', metrics.collect())
+    t.assert_equals(#summary_metrics, 1)
+
+    summary_metrics = utils.find_metric('plain_summary', metrics.collect())
+    t.assert_not(summary_metrics)
+
+end
+
 local test_data_wrong_input = {
     objectives = {error = 'Invalid value for objectives', input = {'summary', nil, {0.5, 0.9, 0.99}}},
     max_age = {error = 'Max age must be positive', input = {'summary', nil, {[0.5]=0.01}, {max_age_time = -1}}},