Move Metrics API spec to Stable (open-telemetry#2104)

Author: reyang

CHANGELOG.md

@@ -33,6 +33,8 @@ release.
 - Add clarifications on how to determine aggregation temporality.
   ([#2013](https://github.com/open-telemetry/opentelemetry-specification/pull/2013),
   [#2032](https://github.com/open-telemetry/opentelemetry-specification/pull/2032))
+- Mark Metrics API spec as Stable.
+  ([#2104](https://github.com/open-telemetry/opentelemetry-specification/pull/2104))
 
 ### Logs
 

specification/metrics/api.md

@@ -1,6 +1,6 @@
 # Metrics API
 
-**Status**: [Feature-freeze](../document-status.md)
+**Status**: [Stable](../document-status.md)
 
 <details>
 <summary>