Add counter:reset method

Author: yngvar-antonsson

CHANGELOG.md

@@ -7,6 +7,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 ### Added
 - collector's method `remove` to clear observations with given label pairs [#263](https://github.com/tarantool/metrics/issues/263)
+- `counter:reset()` method [#260](https://github.com/tarantool/metrics/issues/260)
 
 ## [0.10.0] - 2021-08-03
 ### Changed

doc/monitoring/api_reference.rst

@@ -69,6 +69,10 @@ Counter
 
         Removes an observation with ``label_pairs``.
 
+    .. 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.
 

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

@@ -269,7 +269,6 @@ g.test_global_labels = function()
     t.assert_equals(obs_bucket_inf_hist_2.value, 1, "bucket +inf has 1 value: 2; observation global label has changed")
 end
 
-
 g.test_counter_remove_metric_by_label = function()
     local c = metrics.counter('cnt')
 
@@ -303,3 +302,11 @@ g.test_gauge_remove_metric_by_label = function()
         {'gauge', 1, {label = 2}},
     })
 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