Add counter:reset method

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
 ## [Unreleased]
+### Added
+- `counter:reset()` method [#260](https://github.com/tarantool/metrics/issues/260)
 
 ## [0.10.0] - 2021-08-03
 ### Changed

doc/monitoring/api_reference.rst

@@ -65,6 +65,13 @@ Counter
 
         :rtype: table
 
+    .. method:: reset(label_pairs)
+
+        Set an observation under ``label_pairs`` to 0.
+
+        :param table label_pairs: Table containing label names as keys,
+                                  label values as values.
+
 .. _gauge:
 
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

metrics/collectors/counter.lua

@@ -9,4 +9,8 @@ function Counter:inc(num, label_pairs)
     Shared.inc(self, num, label_pairs)
 end
 
+function Counter:reset(label_pairs)
+    Shared.set(self, 0, label_pairs)
+end
+
 return Counter

test/collectors_test.lua

@@ -268,3 +268,11 @@ g.test_global_labels = function()
         { le = metrics.INF, alias = 'another alias', nlabel = 3 }, observations)
     t.assert_equals(obs_bucket_inf_hist_2.value, 1, "bucket +inf has 1 value: 2; observation global label has changed")
 end
+
+g.test_collector_reset = function()
+    local c = metrics.counter('cnt', 'some counter')
+    c:inc()
+    t.assert_equals(c:collect()[1].value, 1)
+    c:reset()
+    t.assert_equals(c:collect()[1].value, 0)
+end