fix: impact metric serialization and overload

Author: sjaanus

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

@@ -1,7 +1,12 @@
 package io.getunleash.impactmetrics;
 
+import com.google.gson.annotations.SerializedName;
+
 public enum MetricType {
+    @SerializedName("counter")
     COUNTER,
+    @SerializedName("gauge")
     GAUGE,
+    @SerializedName("histogram")
     HISTOGRAM
 }

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

@@ -43,6 +43,10 @@ public void defineGauge(String name, String help) {
         metricRegistry.gauge(new MetricOptions(name, help, labelNames));
     }
 
+    public void defineHistogram(String name, String help) {
+        defineHistogram(name, help, null);
+    }
+
     public void defineHistogram(String name, String help, @Nullable List<Double> buckets) {
         if (name == null || name.isEmpty() || help == null || help.isEmpty()) {
             LOGGER.warn("Histogram name or help cannot be empty: {}, {}", name, help);