Mark In-Memory/OTLP/Stdout exporters as Stable (open-telemetry#2175)

* mark In-Memory/OTLP/Stdout exporters as stable

* update changelog

Author: reyang

CHANGELOG.md

@@ -19,6 +19,8 @@ release.
   ([#2131](https://github.com/open-telemetry/opentelemetry-specification/pull/2131))
 - Remove the concept of supported temporality, keep preferred.
   ([#2154](https://github.com/open-telemetry/opentelemetry-specification/pull/2154))
+- Mark In-memory, OTLP and Stdout exporter specs as Stable.
+  ([#2175](https://github.com/open-telemetry/opentelemetry-specification/pull/2175))
 
 ### Logs
 

specification/metrics/sdk_exporters/in-memory.md

@@ -1,6 +1,6 @@
 # OpenTelemetry Metrics Exporter - In-memory
 
-**Status**: [Feature-freeze](../../document-status.md)
+**Status**: [Stable](../../document-status.md)
 
 In-memory Metrics Exporter is a [Push Metric
 Exporter](../sdk.md#push-metric-exporter) which accumulates metrics data in the

specification/metrics/sdk_exporters/otlp.md

@@ -1,6 +1,6 @@
 # OpenTelemetry Metrics Exporter - OTLP
 
-**Status**: [Feature-freeze](../../document-status.md)
+**Status**: [Stable](../../document-status.md)
 
 OTLP Metrics Exporter is a [Push Metric
 Exporter](../sdk.md#push-metric-exporter) which sends metrics via the

specification/metrics/sdk_exporters/stdout.md

@@ -1,6 +1,6 @@
 # OpenTelemetry Metrics Exporter - Standard output
 
-**Status**: [Feature-freeze](../../document-status.md)
+**Status**: [Stable](../../document-status.md)
 
 "Standard output" Metrics Exporter is a [Push Metric
 Exporter](../sdk.md#push-metric-exporter) which outputs the metrics to