move the lock down in the Diff method

pkg/collector/distinct_string_collector.go

@@ -106,13 +106,14 @@ func (d *DistinctString) Size() int {
 
 // Diff returns all new strings collected since the last time diff was called
 func (d *DistinctString) Diff() ([]string, error) {
-	d.mtx.Lock()
-	defer d.mtx.Unlock()
-
+	// can check diffEnabled without lock because it is not modified after creation
 	if !d.diffEnabled {
 		return nil, errDiffNotEnabled
 	}
 
+	d.mtx.Lock()
+	defer d.mtx.Unlock()
+
 	ss := make([]string, 0, len(d.new))
 
 	for k := range d.new {

pkg/collector/distinct_value_collector.go

@@ -111,13 +111,14 @@ func (d *DistinctValue[T]) Size() int {
 // Diff returns all new strings collected since the last time diff was called
 // returns nil if diff is not enabled
 func (d *DistinctValue[T]) Diff() ([]T, error) {
-	d.mtx.Lock()
-	defer d.mtx.Unlock()
-
+	// can check diffEnabled without lock because it is not modified after creation
 	if !d.diffEnabled {
 		return nil, errDiffNotEnabled
 	}
 
+	d.mtx.Lock()
+	defer d.mtx.Unlock()
+
 	ss := make([]T, 0, len(d.new))
 	for k := range d.new {
 		ss = append(ss, k)

pkg/collector/scoped_distinct_string.go

@@ -89,13 +89,13 @@ func (d *ScopedDistinctString) Exceeded() bool {
 
 // Diff returns all new strings collected since the last time Diff was called
 func (d *ScopedDistinctString) Diff() (map[string][]string, error) {
-	d.mtx.Lock()
-	defer d.mtx.Unlock()
-
 	if !d.diffEnabled {
 		return nil, errDiffNotEnabled
 	}
 
+	d.mtx.Lock()
+	defer d.mtx.Unlock()
+
 	ss := map[string][]string{}
 
 	for k, v := range d.cols {