add STORE_NOMINATED_FOR_SCHEDULED_COMPACTION metric entity

Author: Whitney Deng

internal/venice-client-common/src/main/java/com/linkedin/venice/stats/metrics/MetricType.java

@@ -26,5 +26,10 @@ public enum MetricType {
   /**
    * For Counter: A simple counter that can be added to.
    */
-  COUNTER;
+  COUNTER,
+
+  /**
+   * For UpDownCounter: A counter that can be added to or subtracted from. Emits the absolute value of the metric value.
+   */
+  GAUGE;
 }

services/venice-controller/src/main/java/com/linkedin/venice/controller/stats/ControllerMetricEntity.java

@@ -17,7 +17,13 @@ public enum ControllerMetricEntity {
   REPUSH_STORE_ENDPOINT_CALL_COUNT(
       MetricType.COUNTER, MetricUnit.NUMBER, "Count of all calls to a controller endpoint",
       setOf(VENICE_STORE_NAME, VENICE_CLUSTER_NAME, REPUSH_STORE_TRIGGER_SOURCE, VENICE_RESPONSE_STATUS_CODE_CATEGORY)
-  );
+  ),
+  STORE_NOMINATED_FOR_SCHEDULED_COMPACTION(
+      MetricType.GAUGE, MetricUnit.NUMBER,
+      "When a store is nominated for scheduled compaction and remains uncompacted, this metric will be at 1."
+          + " When the store is compacted, this metric will return to 0.",
+      setOf(VENICE_STORE_NAME, VENICE_CLUSTER_NAME)
+  ),;
 
   private final MetricEntity metricEntity;
   private final String metricName;