public StatefulMetric (#1474)

All the other classes in the hierarchy are public too, this is probably
just an oversight.

---------

Signed-off-by: Ondra Pelech <ondra.pelech@gmail.com>

Author: sideeffffect

prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/metrics/StatefulMetric.java

@@ -28,7 +28,8 @@
  * because in Java <i>synchronous</i> and <i>asynchronous</i> usually refers to multi-threading, but
  * this has nothing to do with multi-threading.
  */
-abstract class StatefulMetric<D extends DataPoint, T extends D> extends MetricWithFixedMetadata {
+public abstract class StatefulMetric<D extends DataPoint, T extends D>
+    extends MetricWithFixedMetadata {
 
   /** Map label values to data points. */
   private final ConcurrentHashMap<List<String>, T> data = new ConcurrentHashMap<>();