Defer call to iso.closeAppend

This is taken from #6918. Since we probably won't merge #6918 before
the relase, we have to do this bit of it as it fixes an actual bug
(iso.closeAppend is not called if the append fails because of an error
logging to the WAL).

Signed-off-by: beorn7 <beorn@grafana.com>

tsdb/head.go

@@ -1045,6 +1045,7 @@ func (a *headAppender) Commit() error {
 	defer a.head.metrics.activeAppenders.Dec()
 	defer a.head.putAppendBuffer(a.samples)
 	defer a.head.putSeriesBuffer(a.sampleSeries)
+	defer a.head.iso.closeAppend(a.appendID)
 
 	if err := a.log(); err != nil {
 		return errors.Wrap(err, "write to WAL")
@@ -1071,7 +1072,6 @@ func (a *headAppender) Commit() error {
 
 	a.head.metrics.samplesAppended.Add(float64(total))
 	a.head.updateMinMaxTime(a.mint, a.maxt)
-	a.head.iso.closeAppend(a.appendID)
 
 	return nil
 }