Add docs

CHANGELOG.md

@@ -13,6 +13,8 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - The `go.opentelemetry.io/otel/semconv/v1.22.0` package.
   The package contains semantic conventions from the `v1.22.0` version of the OpenTelemetry Semantic Conventions. (#4735)
 - Add `WithResourceAsConstantLabels` option to apply resource attributes for every metric emitted by the Prometheus exporter. (#4733)
+- Experimental cardinality limiting is added to the metric SDK.
+  See [metric documentation](./sdk/metric/EXPERIMENTAL.md#cardinality-limit) for more information about this feature and how to enable it. (#4457)
 
 ### Changed
 

sdk/metric/EXPERIMENTAL.md

@@ -0,0 +1,50 @@
+# Experimental Features
+
+The metric SDK contains features that have not yet stabilized in the OpenTelemetry specification.
+These features are added to the OpenTelemetry Go metric SDK prior to stabilization in the specification so that users can start experimenting with them and provide feedback.
+
+These feature may change in backwards incompatible ways as feedback is applied.
+See the [Compatibility and Stability](#compatibility-and-stability) section for more information.
+
+## Features
+
+- [Cardinality Limit](#cardinality-limit)
+
+### Cardinality Limit
+
+The cardinality limit is the hard limit on the number of metric streams that can be collected for a single instrument.
+
+This experimental feature can be enabled by setting the `OTEL_GO_X_CARDINALITY_LIMIT` environment value.
+The value must be an integer value.
+All other values are ignored.
+
+If the value set is less than or equal to `0`, no limit will be applied.
+
+#### Examples
+
+Set the cardinality limit to 2000.
+
+```console
+export OTEL_GO_X_CARDINALITY_LIMIT=2000
+```
+
+Set an infinite cardinality limit (functionally equivalent to disabling the feature).
+
+```console
+export OTEL_GO_X_CARDINALITY_LIMIT=-1
+```
+
+Disable the cardinality limit.
+
+```console
+unset OTEL_GO_X_CARDINALITY_LIMIT
+```
+
+## Compatibility and Stability
+
+Experimental features do not fall within the scope of the OpenTelemetry Go versioning and stability [policy](../../VERSIONING.md).
+These features may be removed or modified in successive version releases, including patch versions.
+
+When an experimental feature is promoted to a stable feature, a migration path will be included in the changelog entry of the release.
+There is no guarantee that any environment variable feature flags that enabled the experimental feature will be supported by the stable version.
+If they are supported, they may be accompanied with a deprecation notice stating a timeline for the removal of that support.