prometheus: avoid preallocating scope attributes when disabled (#8404)

Fixes #8401

A regression test was too verbose + brittle to be useful.

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

CHANGELOG.md

@@ -10,6 +10,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Fixed
 
+- Avoid preallocating scope attributes when they are disabled in `go.opentelemetry.io/otel/exporters/prometheus` . (#8404)
 - Fix memory leak in `go.opentelemetry.io/otel/sdk/metric/exemplar` Reservoir where full `context.Context` was stored, pinning large objects like gRPC transport buffers. (#8389)
 - Interpret HTTP `Retry-After` header values as seconds instead of nanoseconds when retrying OTLP HTTP exports in `go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp`, `go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp`, `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#8383)
 - Drastically reduce histogram heap allocations by reusing `BucketCounts` and `Exemplars` slices across `Collect` cycles in the cumulative histogram aggregation in `go.opentelemetry.io/otel/sdk/metric`. (#8428)

exporters/prometheus/exporter.go

@@ -251,8 +251,11 @@ func (c *collector) Collect(ch chan<- prometheus.Metric) {
 			})
 			continue
 		}
-		// resource attributes + scope attributes + scope name + scope version + scope schema url
-		n := len(c.resourceKeyVals.keys) + 3 + scopeMetrics.Scope.Attributes.Len()
+		// resource attributes + (if enabled) scope fields
+		n := len(c.resourceKeyVals.keys)
+		if !c.disableScopeInfo {
+			n += 3 + scopeMetrics.Scope.Attributes.Len()
+		}
 		kv := keyVals{
 			keys: make([]string, 0, n),
 			vals: make([]string, 0, n),