[hotfix][core] Simplify Histogram

[klistopad]

What is the purpose of the change

The goal of this PR is to simplify code in Histogram class and add absent unit tests for the class. It is refactoring no logic changed.

Brief change log

• 54d101941e3487b302bac11780154372678f687d: Histogram class simplified and added tests for that.

Verifying this change

Unit tests added: HistogramTest

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

[flinkbot]

CI report:

• 54d101941e3487b302bac11780154372678f687d Azure: SUCCESS

Bot commands

The @flinkbot bot supports the following commands:

• @flinkbot run azure re-run the last Azure build