Prepare for v0.21.2 release (#1463)

Backport #1434 and #1452 to v0.21.x.

Author: jtescher

opentelemetry-sdk/CHANGELOG.md

@@ -29,6 +29,8 @@
 
 - [#1375](https://github.com/open-telemetry/opentelemetry-rust/pull/1375/) Fix metric collections during PeriodicReader shutdown
 
+## v0.21.2
+
 ### Fixed
 
 - Fix delta aggregation metric reuse. (#1434)

opentelemetry-sdk/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "opentelemetry_sdk"
-version = "0.21.1"
+version = "0.21.2"
 description = "The SDK for the OpenTelemetry metrics collection and distributed tracing framework"
 homepage = "https://github.com/open-telemetry/opentelemetry-rust"
 repository = "https://github.com/open-telemetry/opentelemetry-rust"