Prepare for the metrics SDK experimental release (#1912)

* prepare for the metrics sdk experiemental release

* update changelog

* update changelog

Author: reyang

CHANGELOG.md

@@ -18,6 +18,17 @@ release.
 
 ### Metrics
 
+- Added Experimental Metrics SDK specification.
+  ([#1673](https://github.com/open-telemetry/opentelemetry-specification/pull/1673),
+  [#1730](https://github.com/open-telemetry/opentelemetry-specification/pull/1730),
+  [#1840](https://github.com/open-telemetry/opentelemetry-specification/pull/1840),
+  [#1842](https://github.com/open-telemetry/opentelemetry-specification/pull/1842),
+  [#1864](https://github.com/open-telemetry/opentelemetry-specification/pull/1864),
+  [#1828](https://github.com/open-telemetry/opentelemetry-specification/pull/1828),
+  [#1888](https://github.com/open-telemetry/opentelemetry-specification/pull/1888),
+  [#1912](https://github.com/open-telemetry/opentelemetry-specification/pull/1912),
+  [#1913](https://github.com/open-telemetry/opentelemetry-specification/pull/1913))
+
 ### Logs
 
 ### Resource

specification/metrics/sdk.md

@@ -2,21 +2,6 @@
 
 **Status**: [Experimental](../document-status.md)
 
-**Owner:**
-
-* [Reiley Yang](https://github.com/reyang)
-
-**Domain Experts:**
-
-* [Bogdan Drutu](https://github.com/bogdandrutu)
-* [Josh Suereth](https://github.com/jsuereth)
-* [Joshua MacDonald](https://github.com/jmacd)
-
-Note: this specification is subject to major changes. To avoid thrusting
-language client maintainers, we don't recommend OpenTelemetry clients to start
-the implementation unless explicitly communicated via
-[OTEP](https://github.com/open-telemetry/oteps#opentelemetry-enhancement-proposal-otep).
-
 <details>
 <summary>
 Table of Contents