[server] SN read quota usage should return NaN for stores with no rep…

…licas (#1157)

If a store's partition count is small it's normal and expectd to have some
hosts in the cluster to not have any replicas assigned to them. The hosts
should emit NaN for the SN read quota usage metric instead of -1. Emitting
-1 or 0 will mess up aggregation of metrics used for monitoring.

services/venice-server/src/main/java/com/linkedin/venice/stats/ServerQuotaUsageStats.java

@@ -65,7 +65,7 @@ public void setTokenBucket(TokenBucket tokenBucket) {
    */
   private Double getReadQuotaUsageRatio() {
     if (tokenBucket == null) {
-      return -1d;
+      return Double.NaN;
     } else {
       return tokenBucket.getStaleUsageRatio();
     }

services/venice-server/src/test/java/com/linkedin/venice/stats/AggServerQuotaUsageStatsTest.java

@@ -70,10 +70,10 @@ public void testAggServerQuotaUsageStats() {
     double expectedUsageRatio = 0.5;
     doReturn(expectedUsageRatio).when(mockTokenBucket).getStaleUsageRatio();
 
-    Assert.assertEquals(metricsRepository.getMetric(readQuotaUsageRatioString).value(), -1d);
+    Assert.assertEquals(metricsRepository.getMetric(readQuotaUsageRatioString).value(), Double.NaN);
     aggServerQuotaUsageStats.setStoreTokenBucket(storeName, mockTokenBucket);
     Assert.assertEquals(metricsRepository.getMetric(readQuotaUsageRatioString).value(), expectedUsageRatio);
     aggServerQuotaUsageStats.setStoreTokenBucket(storeName, null);
-    Assert.assertEquals(metricsRepository.getMetric(readQuotaUsageRatioString).value(), -1d);
+    Assert.assertEquals(metricsRepository.getMetric(readQuotaUsageRatioString).value(), Double.NaN);
   }
 }