Small memory footprint reduction for folly::MultiLevelTimeSeries

Summary:
* Spend some extra time in getNumBuckets() in order to save
space: 8 bytes per object on 64-bit platforms.

Test Plan: Ran the unit tests

Reviewed By: simpkins@fb.com

Subscribers: ruibalp

FB internal diff: D1488345

folly/stats/MultiLevelTimeSeries-defs.h

@@ -26,8 +26,7 @@ MultiLevelTimeSeries<VT, TT>::MultiLevelTimeSeries(
   size_t numBuckets,
   size_t numLevels,
   const TimeType levelDurations[])
-    : numBuckets_(numBuckets),
-      cachedTime_(0),
+    : cachedTime_(0),
       cachedSum_(0),
       cachedCount_(0) {
     CHECK_GT(numLevels, 0);

folly/stats/MultiLevelTimeSeries.h

@@ -73,7 +73,10 @@ class MultiLevelTimeSeries {
   /*
    * Return the number of buckets used to track time series at each level.
    */
-  size_t numBuckets() const { return numBuckets_; }
+  size_t numBuckets() const {
+    // The constructor ensures that levels_ has at least one item
+    return levels_[0].numBuckets();
+  }
 
   /*
    * Return the number of levels tracked by MultiLevelTimeSeries.
@@ -297,7 +300,6 @@ class MultiLevelTimeSeries {
   void flush();
 
  private:
-  size_t numBuckets_;
   std::vector<Level> levels_;
 
   // Updates within the same time interval are cached