feat(go): expose fast writer batch pending metrics

docs/ops/100k-capacity-baseline.md

@@ -76,6 +76,8 @@ systemd gateway 已配置 `LimitNOFILE=1048576`，需要持续保持。
 | NATS bridge | `vehicle_bridge_nats_consumer_pending` | burst 后下降 |
 | NATS bridge | `vehicle_bridge_batch_pending_messages` | 稳态接近 0，burst 后下降 |
 | NATS bridge | `vehicle_bridge_batch_duration_ms_histogram_bucket` | p99 不持续上升 |
+| NATS fast writer | `vehicle_fast_writer_batch_pending_messages` | 稳态接近 0，burst 后下降 |
+| NATS fast writer | `vehicle_fast_writer_batch_pending_envelopes` | 稳态接近 0，burst 后下降 |
 | NATS fast writer | `vehicle_fast_writer_stage_duration_ms_histogram_bucket` | TDengine/Redis/ack 各阶段 p99 不持续上升 |
 | Kafka consumers | `vehicle_*_kafka_lag` | 稳态为 0，burst 后下降 |
 | Realtime Redis | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="redis"}` | p99 毫秒级且不持续上升 |

docs/ops/go-service-observability.md

@@ -47,6 +47,8 @@ curl -fsS http://127.0.0.1:20200/readyz
 | `vehicle_bridge_batch_pending_messages` | NATS messages fetched by the bridge but not yet written to Kafka and acked. |
 | `vehicle_bridge_batch_pending_kafka_messages` | Kafka messages prepared for the current bridge batch. |
 | `vehicle_bridge_batch_duration_ms_histogram` | Bridge batch duration histogram covering Kafka write and NATS ack. Labels: `status`. |
+| `vehicle_fast_writer_batch_pending_messages` | NATS fast writer messages fetched but not yet written to TDengine/Redis and acked. |
+| `vehicle_fast_writer_batch_pending_envelopes` | Valid parsed envelopes in the current NATS fast writer batch. |
 | `vehicle_fast_writer_stage_duration_ms_histogram` | Fast writer stage duration histogram for TDengine, Redis, and NATS ack. Labels: `subject`, `stage`, `status`. |
 | `vehicle_history_writes_total` | TDengine history writes. Labels: `topic`, `status`. |
 | `vehicle_history_batch_pending_messages` | Messages fetched by history writer but not yet flushed and committed. |

docs/ops/go-vehicle-ingest-memory.md

@@ -423,6 +423,15 @@ vehicle_fast_writer_stage_duration_ms_histogram_sum{subject,stage,status}
 
 2026-07-03 后续优化：`nats-fast-writer` 已从“Fetch batch 后逐条写 TDengine”改为“Fetch batch 后先调用 `history.Writer.AppendAllBatch` 批量写 TDengine，再逐条更新 Redis 和 ack”。这样可以减少 TDengine round trip；Redis/ack 仍按单条执行，避免某条实时投影失败时误 ack。
 
+同时新增 batch pending gauge：
+
+```text
+vehicle_fast_writer_batch_pending_messages
+vehicle_fast_writer_batch_pending_envelopes
+```
+
+这两个指标表示 fast-writer 已拉取但尚未完成 TDengine/Redis/ack 的批内压力。带帧率压测时它们应在稳态接近 0，burst 后能够回落；如果持续非 0，需要结合 `vehicle_fast_writer_stage_duration_ms_histogram` 判断卡在 TDengine、Redis 还是 ack。
+
 ### NATS Kafka bridge batch 指标
 
 2026-07-03 已为 NATS -> Kafka bridge 增加 batch pending 和 duration histogram：

docs/ops/vehicle-ingest-runbook.md

@@ -129,6 +129,8 @@ go run ./cmd/load-sim \
 | `vehicle_bridge_nats_consumer_pending` | 持续增长且 `> 10000` | bridge 消费 NATS 的速度跟不上生产速度。 |
 | `vehicle_bridge_batch_pending_messages` | 持续非 0 或 burst 后不回落 | bridge 已拉取 NATS 消息但尚未完成 Kafka 写入和 NATS ack。 |
 | `vehicle_bridge_batch_duration_ms_histogram_bucket` | p99 连续 5 分钟上升 | Kafka 写入或 NATS ack 开始变慢，通常会先于 ack-pending 扩大。 |
+| `vehicle_fast_writer_batch_pending_messages` | 持续非 0 或 burst 后不回落 | fast-writer 已拉取 NATS 消息但尚未完成 TDengine/Redis 写入和 ack。 |
+| `vehicle_fast_writer_batch_pending_envelopes` | 持续非 0 或 burst 后不回落 | fast-writer 当前批次已有有效 envelope 在等待落库或 ack。 |
 | `vehicle_fast_writer_stage_duration_ms_histogram_bucket` | 某个 stage 的 p99 连续 5 分钟上升 | NATS 快速写链路在 TDengine、Redis 或 NATS ack 某一阶段变慢。 |
 | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="redis"}` | p99 连续 5 分钟上升 | Redis 实时投影变慢，会直接影响 realtime consumer 追平能力。 |
 | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="mysql"}` | p99 连续 5 分钟上升 | MySQL 当前态/位置投影变慢，需结合 async queue depth 和 dropped 计数判断。 |

go/vehicle-gateway/cmd/nats-fast-writer/main.go

@@ -262,6 +262,8 @@ func processFastBatch(ctx context.Context, registry *metrics.Registry, appender
 	if len(envelopes) == 0 {
 		return nil
 	}
+	setFastBatchPending(registry, len(messages), len(envelopes))
+	defer setFastBatchPending(registry, 0, 0)
 	subject := fastBatchSubject(validMessages)
 	started := time.Now()
 	err := appender.AppendAllBatch(ctx, envelopes)
@@ -367,6 +369,14 @@ func addFastMetric(registry *metrics.Registry, subject string, status string) {
 	registry.IncCounter("vehicle_fast_writer_messages_total", metrics.Labels{"subject": subject, "status": status})
 }
 
+func setFastBatchPending(registry *metrics.Registry, messages int, envelopes int) {
+	if registry == nil {
+		return
+	}
+	registry.SetGauge("vehicle_fast_writer_batch_pending_messages", nil, float64(messages))
+	registry.SetGauge("vehicle_fast_writer_batch_pending_envelopes", nil, float64(envelopes))
+}
+
 func recordFastWriterStageDuration(registry *metrics.Registry, subject string, stage string, status string, elapsed time.Duration) {
 	if registry == nil {
 		return

go/vehicle-gateway/cmd/nats-fast-writer/main_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"strings"
+	"sync"
 	"testing"
 
 	"lingniu-vehicle-ingest/go/vehicle-gateway/internal/envelope"
@@ -113,11 +114,71 @@ func TestProcessFastBatchAppendsTDengineBatchBeforeRedisAndAck(t *testing.T) {
 	}
 }
 
+func TestProcessFastBatchExposesPendingMetricsDuringAppend(t *testing.T) {
+	first := envelope.FrameEnvelope{Protocol: envelope.ProtocolJT808, VIN: "VIN001", EventID: "evt-6"}
+	second := envelope.FrameEnvelope{Protocol: envelope.ProtocolJT808, VIN: "VIN002", EventID: "evt-7"}
+	firstPayload, err := first.MarshalJSONBytes()
+	if err != nil {
+		t.Fatal(err)
+	}
+	secondPayload, err := second.MarshalJSONBytes()
+	if err != nil {
+		t.Fatal(err)
+	}
+	registry := metrics.NewRegistry()
+	appenderStarted := make(chan struct{})
+	appenderRelease := make(chan struct{})
+	appender := &recordingFastAppender{
+		onAppendBatch: func() {
+			close(appenderStarted)
+			<-appenderRelease
+		},
+	}
+	msgs := []*fastMessage{
+		{subject: "vehicle.raw.go.jt808.v1", data: firstPayload, ack: func() error { return nil }},
+		{subject: "vehicle.raw.go.jt808.v1", data: secondPayload, ack: func() error { return nil }},
+	}
+	var wg sync.WaitGroup
+	wg.Add(1)
+	var processErr error
+	go func() {
+		defer wg.Done()
+		processErr = processFastBatch(context.Background(), registry, appender, &recordingFastUpdater{}, msgs)
+	}()
+	<-appenderStarted
+
+	text := registry.Render()
+	for _, want := range []string{
+		`vehicle_fast_writer_batch_pending_messages 2`,
+		`vehicle_fast_writer_batch_pending_envelopes 2`,
+	} {
+		if !strings.Contains(text, want) {
+			t.Fatalf("pending metric missing %s while appending:\n%s", want, text)
+		}
+	}
+
+	close(appenderRelease)
+	wg.Wait()
+	if processErr != nil {
+		t.Fatalf("processFastBatch() error = %v", processErr)
+	}
+	text = registry.Render()
+	for _, want := range []string{
+		`vehicle_fast_writer_batch_pending_messages 0`,
+		`vehicle_fast_writer_batch_pending_envelopes 0`,
+	} {
+		if !strings.Contains(text, want) {
+			t.Fatalf("pending metric should reset after append, missing %s:\n%s", want, text)
+		}
+	}
+}
+
 type recordingFastAppender struct {
-	count      int
-	batchCount int
-	batchRows  int
-	err        error
+	count         int
+	batchCount    int
+	batchRows     int
+	err           error
+	onAppendBatch func()
 }
 
 func (a *recordingFastAppender) AppendAll(context.Context, envelope.FrameEnvelope) error {
@@ -128,6 +189,9 @@ func (a *recordingFastAppender) AppendAll(context.Context, envelope.FrameEnvelop
 func (a *recordingFastAppender) AppendAllBatch(_ context.Context, envs []envelope.FrameEnvelope) error {
 	a.batchCount++
 	a.batchRows += len(envs)
+	if a.onAppendBatch != nil {
+		a.onAppendBatch()
+	}
 	return a.err
 }