Append flushes instead of setting them in Metrics.Factory#onFlush

core/src/main/java/dev/faststats/Metrics.java

@@ -29,7 +29,7 @@ interface Factory {
         Factory addMetric(Metric<?> metric) throws IllegalArgumentException;
 
         /**
-         * Sets the flush callback for this metrics instance.
+         * Adds a flush callback to this metrics instance.
          * <p>
          * This callback will be invoked when the metrics have been submitted to, and accepted by, the metrics server.
          *

core/src/main/java/dev/faststats/SimpleMetrics.java

@@ -136,7 +136,13 @@ public Factory addMetric(final Metric<?> metric) throws IllegalArgumentException
 
         @Override
         public Factory onFlush(final Runnable flush) {
-            this.flush = flush;
+            final var runnable = this.flush;
+            if (runnable == null) {
+                this.flush = flush;
+            } else this.flush = () -> {
+                runnable.run();
+                flush.run();
+            };
             return this;
         }
     }