streaming: support X-Cache-Hit header

If a value was already available in the local view the request is considered a cache hit.
If the materialized had to wait for a value, it is considered a cache miss.

agent/rpcclient/health/health.go

@@ -42,7 +42,8 @@ func (c *Client) ServiceNodes(
 		if err != nil {
 			return structs.IndexedCheckServiceNodes{}, cache.ResultMeta{}, err
 		}
-		return *result.Value.(*structs.IndexedCheckServiceNodes), cache.ResultMeta{Index: result.Index}, err
+		meta := cache.ResultMeta{Index: result.Index, Hit: result.Cached}
+		return *result.Value.(*structs.IndexedCheckServiceNodes), meta, err
 	}
 
 	out, md, err := c.getServiceNodes(ctx, req)

agent/submatview/materializer.go

@@ -215,9 +215,13 @@ func (m *Materializer) notifyUpdateLocked(err error) {
 	m.updateCh = make(chan struct{})
 }
 
+// Result returned from the View.
 type Result struct {
 	Index uint64
 	Value interface{}
+	// Cached is true if the requested value was already available locally. If
+	// the value is false, it indicates that getFromView had to wait for an update,
+	Cached bool
 }
 
 // getFromView blocks until the index of the View is greater than opts.MinIndex,
@@ -237,6 +241,7 @@ func (m *Materializer) getFromView(ctx context.Context, minIndex uint64) (Result
 	// haven't loaded a snapshot at all yet which means we should wait for one on
 	// the update chan.
 	if result.Index > 0 && result.Index > minIndex {
+		result.Cached = true
 		return result, nil
 	}
 

agent/submatview/store.go

@@ -171,7 +171,7 @@ func (s *Store) Notify(
 			u := cache.UpdateEvent{
 				CorrelationID: correlationID,
 				Result:        result.Value,
-				Meta:          cache.ResultMeta{Index: result.Index},
+				Meta:          cache.ResultMeta{Index: result.Index, Hit: result.Cached},
 			}
 			select {
 			case updateCh <- u: