prevent panic caused by accessing unexported fields

This PR fixes a panic caused by predicate trying to access unexported
fields without properly checking if the field is exported. When this
happens, reflection package panics.

Signed-off-by: Tiago Silva <tiago.silva@goteleport.com>

Author: tigrato

lib.go

@@ -180,6 +180,11 @@ func getFieldByTag(ival interface{}, tagName string, fieldNames []string) (inter
 	valType := val.Type()
 	for i := 0; i < valType.NumField(); i++ {
 		tagValue := valType.Field(i).Tag.Get(tagName)
+		// Always skip unexported fields.
+		// This is important to avoid panics when accessing unexported fields.
+		if !valType.Field(i).IsExported() {
+			continue
+		}
 
 		// If it's an embedded field, traverse it.
 		if tagValue == "" && valType.Field(i).Anonymous {