Set quantile value to nil if there are no samples provided for an age

Closes #303

Author: DifferentialOrange

CHANGELOG.md

@@ -16,6 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Throw an error when http_middelware is processing a wrong handler [#199](https://github.com/tarantool/metrics/issues/199)
 - cartridge issues metric fails before cartridge.cfg() call [#298](https://github.com/tarantool/metrics/issues/298)
 
+### Changed
+- quantile metric is nil if no samples provided for an age [#303](https://github.com/tarantool/metrics/issues/303)
+
 ## [0.10.0] - 2021-08-03
 ### Changed
 - metrics registry refactoring to search with `O(1)` [#188](https://github.com/tarantool/metrics/issues/188)

metrics/quantile.lua

@@ -270,6 +270,11 @@ function quantile.Query(stream_obj, q)
 		-- Fast path when there hasn't been enough data for a flush;
 		-- this also yields better accuracy for small sets of data.
 		local l = stream_obj.b_len
+
+		if l == 0 then
+			return nil
+		end
+
 		local i = math.modf(l * q)
 		stream_obj:maybe_sort()
 		return stream_obj.b[i]

test/quantile_test.lua

@@ -146,7 +146,7 @@ g.test_query_on_empty_quantile = function()
 
     local res = quantile.Query(emptyQuantile, 0.99)
 
-    t.assert_equals(res, math.huge)
+    t.assert_equals(res, nil)
 end
 
 g.test_reset = function()
@@ -156,12 +156,12 @@ g.test_reset = function()
     end
 
     local res = quantile.Query(Quantile, 0.99)
-    t.assert_not_equals(res, math.huge)
+    t.assert_not_equals(res, nil)
 
     quantile.Reset(Quantile)
 
     res = quantile.Query(Quantile, 0.99)
-    t.assert_equals(res, math.huge)
+    t.assert_equals(res, nil)
 end
 
 g.test_quantile_insert_works_after_reset = function()