Move Metrics to its own directory. (#163)

Author: songy23

stats/Metrics.md metrics/Metrics.mdstats/Metrics.md renamed to metrics/Metrics.md

@@ -0,0 +1,8 @@
+# OpenCensus Library Metrics Package
+This documentation serves to document the "look and feel" of the open source metrics package. It 
+describes the key types and the overall behavior.
+
+Note: This is an experimental package and is likely to get backwards-incompatible updates in the future.
+
+## Main APIs
+* [Metrics Data Model](Metrics.md): defines the metrics data model.

metrics/README.md

@@ -13,7 +13,6 @@ metrics.
 data aggregation.
 * [Export API](Export.md): used by the specific vendors to define vendor specific exporters (e.g.
 Prometheus, Stackdriver, SignalFx).
-* [Metrics Data Model](Metrics.md): defines a data model for exporters.
 
 ## Utils
 * [HTTP integration](HTTP.md): document about how to instrument http frameworks.