Fix rangeAdjust to handle cases of non-overlapping time ranges

Summary:
When adjusting nextBucketStart by rounding down, we should also check if
that causes it to not overlap with the user-specified [start, end) time
interval. If it does not overlap, then return an empty return type since
this bucket does not contribute anything to the specified time-range.

Reviewed By: simpkins

Differential Revision: D14121401

fbshipit-source-id: bbfb1d6a71c9fb99244cdff887a7a9ee18741f25

folly/stats/BucketedTimeSeries-defs.h

@@ -491,6 +491,12 @@ ReturnType BucketedTimeSeries<VT, CT>::rangeAdjust(
   // latestTime_.
   if (bucketStart <= latestTime_ && nextBucketStart > latestTime_) {
     nextBucketStart = latestTime_ + Duration(1);
+    // If nextBucketStart is now lower than start then it means that we have
+    // never recorded any data points in the requested time interval,
+    // and can simply return 0.
+    if (start >= nextBucketStart) {
+      return ReturnType{};
+    }
   }
 
   if (start <= bucketStart && end >= nextBucketStart) {