Given a path and a set of buckets, this method will return the value
inside the agg at that path. This is used to extract values for use by
pipeline aggregations (e.g. a derivative might need the price for each
bucket). If the bucket is empty, the configured GapPolicy is invoked to
resolve the missing bucket
agg - A series of agg buckets in the form of a histogram
bucket - A specific bucket that a value needs to be extracted from.
This bucket should be present in the histo
aggPath - The path to a particular value that needs to be extracted.
This path should point to a metric inside the
gapPolicy - The gap policy to apply if empty buckets are found