config: add CacheAge field

This allows operators to tune the caching metadata of responses. In
cases where updaters are split, this value may be useful to adjust
independently from the update period.

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

config/matcher.go

@@ -46,6 +46,12 @@ type Matcher struct {
 	//
 	// A value of 0 disables GC.
 	UpdateRetention int `yaml:"update_retention" json:"update_retention"`
+	// CacheAge controls how long clients should be hinted to cache responses
+	// for.
+	//
+	// If empty, the duration set in "Period" will be used. This means client
+	// may cache "stale" results for 2(Period) - 1 seconds.
+	CacheAge time.Duration `yaml:"cache_age,omitempty" json:"cache_age,omitempty"`
 }
 
 func (m *Matcher) Validate(combo bool) error {
@@ -62,6 +68,9 @@ func (m *Matcher) Validate(combo bool) error {
 	if m.UpdateRetention == 1 || m.UpdateRetention < 0 {
 		m.UpdateRetention = DefaultRetention
 	}
+	if m.CacheAge == 0 {
+		m.CacheAge = m.Period
+	}
 	if !combo {
 		if m.IndexerAddr == "" {
 			return fmt.Errorf("matcher mode requires a remote Indexer address")