Update Appender.Add comment for uint64 refs.
Follow-up to prometheus
@@ -75,7 +75,7 @@ type Appender interface {
// Returned reference numbers are ephemeral and may be rejected in calls
// to AddFast() at any point. Adding the sample via Add() returns a new
// reference number.
- // If the reference is the empty string it must not be used for caching.
+ // If the reference is 0 it must not be used for caching.
Add(l labels.Labels, t int64, v float64) (uint64, error)
// Add adds a sample pair for the referenced series. It is generally faster
