kvserver: add x-region, x-zone Raft msg metrics to Store

Previously, there were no metrics to observe cross-region, cross-zone traffic in
raft messages requests sent / received at each store.

To improve this issue, this commit adds six new store metrics -

```
"raft.rcvd.bytes"
"raft.sent.bytes"
"raft.rcvd.cross_region.bytes"
"raft.sent.cross_region.bytes"
"raft.rcvd.cross_zone.bytes"
"raft.sent.cross_zone.bytes"
```

The first two metrics track the total byte of raft messages received and sent in
a store. Additionally, there are four metrics to track the aggregate byte count
of cross-region, cross-zone Raft messages sent and received in the store.

Note that these metrics capture the byte count of requests immediately upon
message reception and just prior to message transmission. In the case of
messages containing heartbeats or heartbeat_resps, they capture the byte count
of requests with coalesced heartbeats.

To facilitate metrics updating, this commit also introduces a new raft message
handler interface `OutgoingRaftMessageHandler`. This interface captures outgoing
messages right before they are sent to `raftSendQueue`. Note that the message
may not be successfully queued if the outgoing queue is full.

Resolves: #103983

Release note (ops change): Six new metrics -
"raft.rcvd.bytes"
"raft.sent.bytes"
"raft.rcvd.cross_region.bytes"
"raft.sent.cross_region.bytes"
"raft.rcvd.cross_zone.bytes"
"raft.sent.cross_zone.bytes" - are now added to store metrics.

For accurate metrics, follow these assumptions:
- Configure region and zone tier keys consistently across nodes.
- Within a node locality, ensure unique region and zone tier keys.
- Maintain consistent configuration of region and zone tiers across nodes.
- Cross-region but same zone activities should be impossible.

pkg/kv/kvserver/client_raft_helpers_test.go

@@ -172,6 +172,50 @@ func (h *unreliableRaftHandler) HandleDelegatedSnapshot(
 	return h.IncomingRaftMessageHandler.HandleDelegatedSnapshot(ctx, req)
 }
 
+type filterRaftHandlerFuncs struct {
+	// If set to nil, all incoming messages are dropped. If non-nil, returning
+	// true can prevent the message from being dropped.
+	filterReq func(*kvserverpb.RaftMessageRequest) bool
+	// If set to nil, no additional processing is applied to outgoing messages
+	// from the sender's end. If non-nil, returning true can allow the additional
+	// processing.
+	filterReqSent func(*kvserverpb.RaftMessageRequest) bool
+}
+
+// filterRaftHandler applies the filter functions within filterRaftHandlerFuncs
+// to the incoming and outgoing messages. It ensures that only messages with
+// filters that evaluate to true are forwarded to the underlying store
+// interface.
+type filterRaftHandler struct {
+	kvserver.IncomingRaftMessageHandler
+	kvserver.OutgoingRaftMessageHandler
+	filterRaftHandlerFuncs
+}
+
+var _ kvserver.IncomingRaftMessageHandler = &filterRaftHandler{}
+var _ kvserver.OutgoingRaftMessageHandler = &filterRaftHandler{}
+
+func (f *filterRaftHandler) HandleRaftRequest(
+	ctx context.Context,
+	req *kvserverpb.RaftMessageRequest,
+	respStream kvserver.RaftMessageResponseStream,
+) *kvpb.Error {
+	if f.filterReq == nil || !f.filterReq(req) {
+		return nil
+	}
+
+	return f.IncomingRaftMessageHandler.HandleRaftRequest(ctx, req, respStream)
+}
+
+func (f *filterRaftHandler) HandleRaftRequestSent(
+	ctx context.Context, req *kvserverpb.RaftMessageRequest,
+) {
+	if f.filterReqSent == nil || !f.filterReqSent(req) {
+		return
+	}
+	f.OutgoingRaftMessageHandler.HandleRaftRequestSent(ctx, req)
+}
+
 // testClusterStoreRaftMessageHandler exists to allows a store to be stopped and
 // restarted while maintaining a partition using an unreliableRaftHandler.
 type testClusterStoreRaftMessageHandler struct {