Some different histogram implementations:
- https://github.com/HdrHistogram/HdrHistogram
- https://github.com/tdunning/t-digest
- https://github.com/dropwizard/metrics/blob/master/metrics-core/src/main/java/com/codahale/metrics/Histogram.java
- https://hadoop.apache.org/docs/current/api/org/apache/hadoop/mapreduce/lib/aggregate/ValueHistogram.html
- https://github.com/twitter/ostrich/blob/master/src/main/scala/com/twitter/ostrich/stats/Histogram.scala
- https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/EstimatedHistogram.java
- https://github.com/apache/cassandra/blob/trunk/src/java/org/apache/cassandra/utils/StreamingHistogram.java