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 #25151

(cherry picked from commit 176fca2)

closes #25152

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