sdk/metric: Add Unit Tests for Cardinality Limits (#7164)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/6978
Towards https://github.com/open-telemetry/opentelemetry-go/issues/6887

## What
 
- Unit tests cover all scenarios for cardinality limits.
- Tests validate configuration, enforcement, and default behavior.

## Notes

I've added more user-oriented usage tests. The targeted unit tests
already exist in the
`sdk/metric/internal/aggregate/_test` files.

---------

Co-authored-by: Tyler Yahn <MrAlias@users.noreply.github.com>
Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -41,7 +41,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
   - `RPCGRPCResponseMetadata`
 - Add `ErrorType` attribute helper function to the `go.opentelmetry.io/otel/semconv/v1.34.0` package. (#6962)
 - Add `WithAllowKeyDuplication` in `go.opentelemetry.io/otel/sdk/log` which can be used to disable deduplication for log records. (#6968)
-- Add `WithCardinalityLimit` option to configure the cardinality limit in `go.opentelemetry.io/otel/sdk/metric`. (#6996, #7065, #7081)
+- Add `WithCardinalityLimit` option to configure the cardinality limit in `go.opentelemetry.io/otel/sdk/metric`. (#6996, #7065, #7081, #7164)
 - Add `Clone` method to `Record` in `go.opentelemetry.io/otel/log` that returns a copy of the record with no shared state. (#7001)
 - The `go.opentelemetry.io/otel/semconv/v1.36.0` package.
   The package contains semantic conventions from the `v1.36.0` version of the OpenTelemetry Semantic Conventions.

sdk/metric/provider_test.go

@@ -174,3 +174,73 @@ func TestMeterProviderMixingOnRegisterErrors(t *testing.T) {
 		"Metrics produced for instrument collected by different MeterProvider",
 	)
 }
+
+func TestMeterProviderCardinalityLimit(t *testing.T) {
+	const uniqueAttributesCount = 10
+
+	tests := []struct {
+		name           string
+		options        []Option
+		wantDataPoints int
+	}{
+		{
+			name:           "no limit (default)",
+			options:        nil,
+			wantDataPoints: uniqueAttributesCount,
+		},
+		{
+			name:           "no limit (limit=0)",
+			options:        []Option{WithCardinalityLimit(0)},
+			wantDataPoints: uniqueAttributesCount,
+		},
+		{
+			name:           "no limit (negative)",
+			options:        []Option{WithCardinalityLimit(-5)},
+			wantDataPoints: uniqueAttributesCount,
+		},
+		{
+			name:           "limit=5",
+			options:        []Option{WithCardinalityLimit(5)},
+			wantDataPoints: 5,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			reader := NewManualReader()
+
+			opts := append(tt.options, WithReader(reader))
+			mp := NewMeterProvider(opts...)
+
+			meter := mp.Meter("test-meter")
+			counter, err := meter.Int64Counter("metric")
+			require.NoError(t, err, "failed to create counter")
+
+			for i := range uniqueAttributesCount {
+				counter.Add(
+					context.Background(),
+					1,
+					api.WithAttributes(attribute.Int("key", i)),
+				)
+			}
+
+			var rm metricdata.ResourceMetrics
+			err = reader.Collect(context.Background(), &rm)
+			require.NoError(t, err, "failed to collect metrics")
+
+			require.Len(t, rm.ScopeMetrics, 1, "expected 1 ScopeMetrics")
+			require.Len(t, rm.ScopeMetrics[0].Metrics, 1, "expected 1 Metric")
+
+			data := rm.ScopeMetrics[0].Metrics[0].Data
+			require.IsType(t, metricdata.Sum[int64]{}, data, "expected metricdata.Sum[int64]")
+
+			sumData := data.(metricdata.Sum[int64])
+			assert.Len(
+				t,
+				sumData.DataPoints,
+				tt.wantDataPoints,
+				"unexpected number of data points",
+			)
+		})
+	}
+}