fix: drop negative counter inc (#366)

Author: kwasniew

src/main/java/io/getunleash/impactmetrics/CounterImpl.java

@@ -28,7 +28,7 @@ public void inc(long value) {
 
     @Override
     public void inc(long value, Map<String, String> labels) {
-        if (value == 0) {
+        if (value <= 0) {
             return;
         }
         String key = getLabelKey(labels);

src/test/java/io/getunleash/impactmetrics/MetricTypeTest.java

@@ -33,6 +33,7 @@ public void should_increment_with_custom_value_and_labels() {
         Counter counter = registry.counter(new MetricOptions("labeled_counter", "with labels"));
 
         counter.inc(3, Map.of("foo", "bar"));
+        counter.inc(-1, Map.of("foo", "bar"));
         counter.inc(2, Map.of("foo", "bar"));
 
         List<CollectedMetric> metrics = registry.collect();