hotfix NullStatsdExporterClient

2.5.1

hotfix add histogram to NullStatsdExporterClient

Add Histogram method to interface and MetricCollector

2.5.0

add new histogram method to metric collector (#15)

Add Histogram type

add histogram method and fix summary (#14)

* add histogram method

* add comments

3.0.2

add new gauges to MetricCollector (#13)

Downgrade major version

2.3.0

add new gauges to MetricCollector (#13)

(MAJOR) add new gauge methods

3.0.1

fix comments

2.2.0: fix type for microseconds (#12)

* fix type for microseconds