feat(util/dcfilter): allow for creating stand-alone filter (#4061)

Sometimes we don't have access to all dcs right away,
so it's more convenient to use Check instead of Apply.
It also allows for parsing dc filters just once.

v3/pkg/util/inexlist/dcfilter/dcfilter.go

@@ -37,6 +37,29 @@ func Apply(dcMap map[string][]string, filters []string) ([]string, error) {
 	return filtered, nil
 }
 
+// Filter that lets you filter datacenters.
+type Filter struct {
+	filters []string
+	inex    inexlist.InExList
+}
+
+func NewFilter(filters []string) (*Filter, error) {
+	// Decorate filters and create inexlist
+	inex, err := inexlist.ParseInExList(decorate(filters))
+	if err != nil {
+		return nil, errors.Wrapf(err, "parse dc filter %v", filters)
+	}
+	return &Filter{
+		filters: filters,
+		inex:    inex,
+	}, nil
+}
+
+// Check returns true iff dc matches filter.
+func (f *Filter) Check(dc string) bool {
+	return len(f.inex.Filter([]string{dc})) > 0
+}
+
 func decorate(filters []string) []string {
 	if len(filters) == 0 {
 		filters = append(filters, "*")