Merge pull request #53 from brancz/cut-1.2

Add CHANGELOG.md entry for v1.2.0

CHANGELOG.md

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+# Changelog
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
+and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [1.2.0](https://github.com/grpc-ecosystem/go-grpc-prometheus/releases/tag/v1.2.0) - 2018-06-04
+
+### Added
+
+* Provide metrics object as `prometheus.Collector`, for conventional metric registration.
+* Support non-default/global Prometheus registry.
+* Allow configuring counters with `prometheus.CounterOpts`.
+
+### Changed
+
+* Remove usage of deprecated `grpc.Code()`.
+* Remove usage of deprecated `grpc.Errorf` and replace with `status.Errorf`.
+
+---
+
+This changelog was started with version `v1.2.0`, for earlier versions refer to the respective [GitHub releases](https://github.com/grpc-ecosystem/go-grpc-prometheus/releases).