Move metrics SDK spec to Mixed

CHANGELOG.md

@@ -128,6 +128,9 @@ release.
   ([#2325](https://github.com/open-telemetry/opentelemetry-specification/pull/2325))
 - Changed the Exemplar wording, exemplar should be turned off by default.
   ([#2414](https://github.com/open-telemetry/opentelemetry-specification/pull/2414))
+- Mark Metrics SDK spec as Mixed, with most components moving to Stable, while
+  Exemplar remaining Feature-freeze.
+  ([#2304](https://github.com/open-telemetry/opentelemetry-specification/pull/2304))
 
 ### Logs
 

specification/metrics/sdk.md

@@ -1,6 +1,6 @@
 # Metrics SDK
 
-**Status**: [Feature-freeze](../document-status.md)
+**Status**: [Mixed](../document-status.md)
 
 <details>
 <summary>Table of Contents</summary>
@@ -49,6 +49,8 @@
 
 ## MeterProvider
 
+**Status**: [Stable](../document-status.md)
+
 A `MeterProvider` MUST provide a way to allow a [Resource](../resource/sdk.md) to
 be specified. If a `Resource` is specified, it SHOULD be associated with all the
 metrics produced by any `Meter` from the `MeterProvider`. The [tracing SDK
@@ -183,9 +185,10 @@ are the inputs:
     apply a [default aggregation](#default-aggregation) configurable on the
     basis of instrument kind according to the [MetricReader](#metricreader)
     instance.
-  * The `exemplar_reservoir` (optional) to use for storing exemplars.
-    This should be a factory or callback similar to aggregation which allows
-    different reservoirs to be chosen by the aggregation.
+  * **Status**: [Feature-freeze](../document-status.md) - the
+    `exemplar_reservoir` (optional) to use for storing exemplars. This should be
+    a factory or callback similar to aggregation which allows different
+    reservoirs to be chosen by the aggregation.
 
 In order to avoid conflicts, views which specify a name SHOULD have an
 instrument selector that selects at most one instrument. For the registration
@@ -467,13 +470,17 @@ between two non-identical `Metric` instances having the same `name`:
 
 ## Attribute limits
 
+**Status**: [Stable](../document-status.md)
+
 Attributes which belong to Metrics are exempt from the
 [common rules of attribute limits](../common/common.md#attribute-limits) at this
 time. Attribute truncation or deletion could affect identity of metric time
 series and the topic requires further analysis.
 
 ## Exemplar
 
+**Status**: [Feature-freeze](../document-status.md)
+
 Exemplars are example data points for aggregated data. They provide specific
 context to otherwise general aggregations. Exemplars allow correlation between
 aggregated metric data and the original API calls where measurements are
@@ -635,6 +642,8 @@ measurements using the equivalent of the following naive algorithm:
 
 ## MetricReader
 
+**Status**: [Stable](../document-status.md)
+
 `MetricReader` is an SDK implementation object that provides the
 common configurable aspects of the OpenTelemetry Metrics SDK and
 determines the following capabilities:
@@ -763,6 +772,8 @@ from `MetricReader` and start a background task which calls the inherited
 
 ## MetricExporter
 
+**Status**: [Stable](../document-status.md)
+
 `MetricExporter` defines the interface that protocol-specific exporters MUST
 implement so that they can be plugged into OpenTelemetry SDK and support sending
 of telemetry data.
@@ -994,6 +1005,8 @@ errors/exceptions are taken care of.
 
 ## Compatibility requirements
 
+**Status**: [Stable](../document-status.md)
+
 All the metrics components SHOULD allow new methods to be added to existing
 components without introducing breaking changes.
 
@@ -1002,6 +1015,8 @@ existing methods without introducing breaking changes, if possible.
 
 ## Concurrency requirements
 
+**Status**: [Stable](../document-status.md)
+
 For languages which support concurrent execution the Metrics SDKs provide
 specific guarantees and safeties.