fix: prevent an infinite loop in measurementFieldSetChangeMgr (#25155)

The measurementFieldSetChangeMgr has a possibly infinite loop
if the writeRequests channel is closed while in the inner
loop to consolidate write requests. We need to check for ok
on channel receive and exit the loop when ok is false.

closes https://github.com/influxdata/influxdb/issues/25151

tsdb/shard.go

@@ -2020,10 +2020,12 @@ func (fscm *measurementFieldSetChangeMgr) appendToChangesFile(first writeRequest
 	// requests
 	for {
 		select {
-		case wr := <-fscm.writeRequests:
-			changes = append(changes, wr.changes)
-			errorChannels = append(errorChannels, wr.errorReturn)
-			continue
+		case wr, ok := <-fscm.writeRequests:
+			if ok {
+				changes = append(changes, wr.changes)
+				errorChannels = append(errorChannels, wr.errorReturn)
+				continue
+			}
 		default:
 		}
 		break