metrics: Return a copy of LabelFilter in WithEnabledLabels(nil)

LabelFilter.WithEnabledLabels behaved in the same way when passed nil or an
empty slice - it disabled all labels. However, it's convenient to have a "no-op"
option and just return a copy of the original filter. Let's do that when the
passed slice is nil.

This change doesn't affect users. Currently the method is always called with
a non-nil slice.

While here, I documented LabelFilter.WithEnabledLabels with edge cases.

Signed-off-by: Anna Kapuscinska <[email protected]>

pkg/metrics/labelfilter.go

@@ -3,10 +3,22 @@
 
 package metrics
 
+import (
+	"maps"
+)
+
 type LabelFilter map[string]bool
 
+// WithEnabledLabels returns a new LabelFilter with only the labels in enabledLabels enabled.
+// If enabledLabels is nil, a copy of the original LabelFilter is returned.
+// If enabledLabels is empty, all labels are disabled.
+// If enabledLabels contains labels that are not in the original LabelFilter, they are ignored.
 func (f LabelFilter) WithEnabledLabels(enabledLabels []string) LabelFilter {
-	labelFilter := make(LabelFilter)
+	labelFilter := maps.Clone(f)
+	if enabledLabels == nil {
+		return labelFilter
+	}
+
 	// disable all configurable labels
 	for l := range f {
 		labelFilter[l] = false