Merge pull request #3931 from prometheus/cut200

*: cut v2.2.0

CHANGELOG.md

-## 2.2.0-rc.1 / 2018-02-21
+## 2.2.0 / 2018-03-08
 
 * [CHANGE] Rename file SD mtime metric.
 * [CHANGE] Send target update on empty pod IP in Kubernetes SD.
 * [FEATURE] Add API endpoint for flags.
+* [FEATURE] Add API endpoint for dropped targets.
+* [FEATURE] Display annotations on alerts page.
+* [FEATURE] Add option to skip head data when taking snapshots.
 * [ENHANCEMENT] Federation performance improvement.
 * [ENHANCEMENT] Read bearer token file on every scrape.
 * [ENHANCEMENT] Improve typeahead on `/graph` page.
@@ -10,6 +13,7 @@
 * [ENHANCEMENT] Set consul server default to `localhost:8500`.
 * [ENHANCEMENT] Add dropped Alertmanagers to API info endpoint.
 * [ENHANCEMENT] Add OS type meta label to Azure SD.
+* [ENHANCEMENT] Validate required fields in SD configuration.
 * [BUGFIX] Prevent stack overflow on deep recursion in TSDB.
 * [BUGFIX] Correctly read offsets in index files that are greater than 4GB.
 * [BUGFIX] Fix scraping behavior for empty labels.

VERSION

-2.2.0-rc.1
+2.2.0