kvserver: apply kv.raft.command.max_size to outgoing raft msg batching

In #71050, we saw evidence of very large (2.3+GiB) Raft messages being
sent on the stream, which overwhelmed both the sender and the receiver.
Raft messages are batched up before sending and so what must have
happened here is that a large number of reasonably-sized messages (up to
64MiB in this case due to the max_size setting) were merged into a giant
blob. As of this commit, we apply the max_size chunking on the batching
step before sending messages as well.

Closes #71050.

Release note: None

pkg/kv/kvserver/raft_transport.go

@@ -487,15 +487,18 @@ func (t *RaftTransport) processQueue(
 		case err := <-errCh:
 			return err
 		case req := <-ch:
+			maxBytes := MaxCommandSize.Get(&t.st.SV)
 			batch.Requests = append(batch.Requests, *req)
 			req.release()
-			// Pull off as many queued requests as possible.
-			//
-			// TODO(peter): Think about limiting the size of the batch we send.
+			// Pull off as many queued requests as possible, within reason.
 			for done := false; !done; {
 				select {
 				case req = <-ch:
 					batch.Requests = append(batch.Requests, *req)
+					maxBytes -= int64(req.Size())
+					if maxBytes <= 0 {
+						done = true
+					}
 					req.release()
 				default:
 					done = true