limiting: Make SemaphoreLimiter.Acquire() zero-alloc

Previously, `Acquire` on exising key did 1 allocation because
`func() { sem.Release() }` was a closure capturing different variables.

Signed-off-by: Evgenii Stratonikov <e.stratonikov@yadro.com>

limiting/limiter.go

@@ -69,7 +69,7 @@ func (lr *SemaphoreLimiter) Acquire(key string) (ReleaseFunc, bool) {
 	}
 
 	if ok := sem.Acquire(); ok {
-		return func() { sem.Release() }, true
+		return sem.Release, true
 	}
 	return nil, false
 }

limiting/limiter_test.go

@@ -136,3 +136,24 @@ func resetFailCounts(testCases []*testCase) {
 		tc.failCount.Store(0)
 	}
 }
+
+func TestLimiterZeroAlloc(t *testing.T) {
+	const key = "key"
+
+	s, err := limiting.NewSemaphoreLimiter([]limiting.KeyLimit{{Keys: []string{key}, Limit: 1}})
+	require.NoError(t, err)
+
+	testAllocs := func(t *testing.T, key string) float64 {
+		return testing.AllocsPerRun(100, func() {
+			release, ok := s.Acquire(key)
+			require.True(t, ok)
+			release()
+		})
+	}
+	t.Run("existing key", func(t *testing.T) {
+		require.Zero(t, testAllocs(t, key))
+	})
+	t.Run("missing key", func(t *testing.T) {
+		require.Zero(t, testAllocs(t, "missing "+key))
+	})
+}