sdk/metric: Deprecate the sdk/metric/x Feature Supporting Cardinality Limits (#7166)

Fixes #6980
Towards #6887

## What

- Any references to sdk/metric/x in documentation, tests, or examples
are updated to redirect users to use the cardinality limit feature in
the SDK.

---------

Co-authored-by: Robert Pająk <[email protected]>

Author: ysolomchenko

CHANGELOG.md

@@ -60,6 +60,10 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 - Change `SDKProcessorLogQueueCapacity`, `SDKProcessorLogQueueSize`, `SDKProcessorSpanQueueSize`, and `SDKProcessorSpanQueueCapacity` in `go.opentelemetry.io/otel/semconv/v1.36.0/otelconv` to use a `Int64ObservableUpDownCounter`. (#7041)
 - Change `DefaultExemplarReservoirProviderSelector` in `go.opentelemetry.io/otel/sdk/metric` to use `runtime.GOMAXPROCS(0)` instead of `runtime.NumCPU()` for the `FixedSizeReservoirProvider` default size. (#7094)
 
+### Deprecated
+
+- Deprecate support for `OTEL_GO_X_CARDINALITY_LIMIT` environment variable in `go.opentelemetry.io/otel/sdk/metric`. Use `WithCardinalityLimit` option instead. (#7166)
+
 ### Fixed
 
 - Fix `go.opentelemetry.io/otel/exporters/prometheus` to deduplicate suffixes if already present in metric name when UTF8 is enabled. (#7088)

sdk/metric/internal/x/README.md

@@ -8,40 +8,9 @@ See the [Compatibility and Stability](#compatibility-and-stability) section for
 
 ## Features
 
-- [Cardinality Limit](#cardinality-limit)
 - [Exemplars](#exemplars)
 - [Instrument Enabled](#instrument-enabled)
 
-### 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
-```
-
 ### Exemplars
 
 A sample of measurements made may be exported directly as a set of exemplars.

sdk/metric/internal/x/x.go

@@ -10,32 +10,16 @@ package x // import "go.opentelemetry.io/otel/sdk/metric/internal/x"
 import (
 	"context"
 	"os"
-	"strconv"
 )
 
-// CardinalityLimit is an experimental feature flag that defines if
-// cardinality limits should be applied to the recorded metric data-points.
-//
-// To enable this feature set the OTEL_GO_X_CARDINALITY_LIMIT environment
-// variable to the integer limit value you want to use.
-//
-// Setting OTEL_GO_X_CARDINALITY_LIMIT to a value less than or equal to 0
-// will disable the cardinality limits.
-var CardinalityLimit = newFeature("CARDINALITY_LIMIT", func(v string) (int, bool) {
-	n, err := strconv.Atoi(v)
-	if err != nil {
-		return 0, false
-	}
-	return n, true
-})
-
 // Feature is an experimental feature control flag. It provides a uniform way
 // to interact with these feature flags and parse their values.
 type Feature[T any] struct {
 	key   string
 	parse func(v string) (T, bool)
 }
 
+//nolint:unused
 func newFeature[T any](suffix string, parse func(string) (T, bool)) Feature[T] {
 	const envKeyRoot = "OTEL_GO_X_"
 	return Feature[T]{

sdk/metric/internal/x/x_test.go

@@ -7,19 +7,9 @@ import (
 	"testing"
 
 	"github.com/stretchr/testify/assert"
-	"github.com/stretchr/testify/require"
 )
 
-func TestCardinalityLimit(t *testing.T) {
-	const key = "OTEL_GO_X_CARDINALITY_LIMIT"
-	require.Equal(t, key, CardinalityLimit.Key())
-
-	t.Run("100", run(setenv(key, "100"), assertEnabled(CardinalityLimit, 100)))
-	t.Run("-1", run(setenv(key, "-1"), assertEnabled(CardinalityLimit, -1)))
-	t.Run("false", run(setenv(key, "false"), assertDisabled(CardinalityLimit)))
-	t.Run("empty", run(assertDisabled(CardinalityLimit)))
-}
-
+//nolint:unused
 func run(steps ...func(*testing.T)) func(*testing.T) {
 	return func(t *testing.T) {
 		t.Helper()
@@ -29,10 +19,12 @@ func run(steps ...func(*testing.T)) func(*testing.T) {
 	}
 }
 
+//nolint:unused
 func setenv(k, v string) func(t *testing.T) {
 	return func(t *testing.T) { t.Setenv(k, v) }
 }
 
+//nolint:unused
 func assertEnabled[T any](f Feature[T], want T) func(*testing.T) {
 	return func(t *testing.T) {
 		t.Helper()
@@ -44,6 +36,7 @@ func assertEnabled[T any](f Feature[T], want T) func(*testing.T) {
 	}
 }
 
+//nolint:unused
 func assertDisabled[T any](f Feature[T]) func(*testing.T) {
 	var zero T
 	return func(t *testing.T) {