Add comment for MemoryUsage.timestamp epoch millis rounding

Author: dimitris-athanasiou

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/stats/MemoryUsage.java

@@ -54,6 +54,8 @@ private static ConstructingObjectParser<MemoryUsage, Void> createParser(boolean
 
     public MemoryUsage(String jobId, Instant timestamp, long peakUsageBytes) {
         this.jobId = Objects.requireNonNull(jobId);
+        // We intend to store this timestamp in millis granularity. Thus we're rounding here to ensure
+        // internal representation matches toXContent
         this.timestamp = Instant.ofEpochMilli(ExceptionsHelper.requireNonNull(timestamp, TIMESTAMP).toEpochMilli());
         this.peakUsageBytes = peakUsageBytes;
     }