add release notes

Author: digitalsadhu

CHANGELOG.md

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+# Changelog
+
+Notable changes to this project will be documented in this file.
+
+The latest version of this document is always available in [releases][releases-url].
+
+## [Unreleased]
+
+## [2.0.0] - 2018-11-22
+
+-   prom-client global state removed
+-   .metrics() method added
+-   .contentType() method added
+-   lint fixing
+
+[unreleased]: https://github.schibsted.io/finn/express-backstage-pages/compare/v2.0.0...HEAD
+[2.0.0]: https://github.schibsted.io/finn/express-backstage-pages/compare/v1.0.0...v2.0.0
+[releases-url]: https://github.schibsted.io/finn/express-backstage-pages/blob/master/CHANGELOG.md

lib/release-notes/2.0.0.md

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+# v2.0.0 Release notes
+
+## Description
+
+This release is centered around removing global state.
+
+## tl;dr
+
+Changes in this release:
+
+-   prom-client global state removed
+-   .metrics() method added
+-   .contentType() method added
+-   lint fixing
+
+Usage of this module changes from:
+
+```js
+const promClient = require('prom-client');
+const metricsConsumer = new PrometheusConsumer({ client: promClient });
+
+client.pipe(metricsConsumer);
+
+app.get('/metrics', (req, res) => {
+    res.set('Content-Type', promClient.register.contentType).send(
+        promClient.register.metrics(),
+    );
+});
+```
+
+to:
+
+```js
+const promClient = require('prom-client');
+const metricsConsumer = new PrometheusConsumer({ client: promClient });
+
+client.pipe(metricsConsumer);
+
+app.get('/metrics', (req, res) => {
+    res.set('Content-Type', metricsConsumer.contentType()).send(
+        metricsConsumer.metrics(),
+    );
+});
+```
+
+## Changes
+
+This release has the following changes
+
+### global state removal
+
+In previous versions, this module used prom-client's global registry to hold all collected metrics. This had the potential to collide with other usages of prom-client.
+In this version, a new isolated registry is created for each consumer instance.
+
+### .metrics() and .contentType() methods added
+
+In order to support using an isolated registry it was necessary to add a method to extract and render gathered metrics for scraping.
+Instead of rendering metrics out of the global registry you can call `.metrics()`. In order to set the correct content type for the content you can use `.contentType()`
+
+_Example_
+
+```js
+app.get('/metrics', (req, res) => {
+    res.set('Content-Type', consumer.contentType()).send(consumer.metrics());
+});
+```
+
+### lint cleanup
+
+Several files were linted to AirBnB standard