Get point-in-time TDigest snapshot from BasicQuantileStat

Summary:
This diff enables aggregation of point-in-time snapshots of the `TDigest`s that represents the state of `BasicQuantileStat`

We capture a `Snapshot` of `TDigest` and `SlidingWindow<TDigest>`.

The diff exposes `TDigest` and `SlidingWindow<TDigest>` to `QuantileStats`.

These objects are wrapped by the structs, `Snapshot` and `SnapshotEntry`, in `BasicQuantileStat` and `BasicQuantileStatMap`, respectively .

Reviewed By: yfeldblum

Differential Revision: D25170883

fbshipit-source-id: 0201a05cc4020851b652ed3a7b602d5b22aa63be

folly/stats/QuantileEstimator.h

@@ -49,6 +49,11 @@ class SimpleQuantileEstimator {
   /// Flush buffered values
   void flush() { bufferedDigest_.flush(); }
 
+  // Get point-in-time TDigest
+  TDigest getDigest(TimePoint now = ClockT::now()) {
+    return bufferedDigest_.get(now);
+  }
+
  private:
   detail::BufferedDigest<TDigest, ClockT> bufferedDigest_;
 };
@@ -75,6 +80,11 @@ class SlidingWindowQuantileEstimator {
   /// Flush buffered values
   void flush() { bufferedSlidingWindow_.flush(); }
 
+  // Get point-in-time TDigest
+  TDigest getDigest(TimePoint now = ClockT::now()) {
+    return TDigest::merge(bufferedSlidingWindow_.get(now));
+  }
+
  private:
   detail::BufferedSlidingWindow<TDigest, ClockT> bufferedSlidingWindow_;
 };