feat(go): expose fast writer stage latency histogram

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

@@ -76,6 +76,7 @@ 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_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 |
 | Realtime MySQL | `vehicle_realtime_store_update_duration_ms_histogram_bucket{store="mysql"}` | p99 不持续上升 |

docs/ops/go-service-observability.md

@@ -47,6 +47,7 @@ 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_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. |
 | `vehicle_history_batch_pending_rows` | Parsed raw envelopes waiting in the current TDengine batch. |

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

@@ -409,6 +409,18 @@ vehicle_async_sink_queue_depth{sink}
 
 这些指标是进入带帧率压测前的关键保护栏：如果 `queue_depth` 持续增长、`enqueue_total{status="timeout"}` 增长，或 `vehicle_async_sink_publish_duration_ms_histogram_bucket` 的 p99 持续上升，说明入口 publish 队列或下游 NATS/Kafka 写入已成为瓶颈。
 
+### NATS fast writer stage histogram
+
+2026-07-03 已为 NATS fast writer 增加阶段耗时 histogram：
+
+```text
+vehicle_fast_writer_stage_duration_ms_histogram_bucket{subject,stage,status,le}
+vehicle_fast_writer_stage_duration_ms_histogram_count{subject,stage,status}
+vehicle_fast_writer_stage_duration_ms_histogram_sum{subject,stage,status}
+```
+
+`stage` 取值包括 `tdengine`、`redis`、`ack`，分别对应快速写 TDengine raw/location、Redis realtime 投影、NATS ack。带帧率压测时如果某个 stage 的 p99 持续上升，可以直接定位 fast path 是存储瓶颈还是 ack/网络瓶颈。
+
 ### 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 +129,7 @@ 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_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。 |
 | `vehicle_stat_write_duration_ms_histogram_bucket` | p99 连续 5 分钟上升 | MySQL 每日里程统计写入变慢，可能导致 stat Kafka lag 增长。 |

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

@@ -213,7 +213,7 @@ func runFastWorker(ctx context.Context, logger *slog.Logger, registry *metrics.R
 		for _, msg := range msgs {
 			msg := msg
 			operationCtx, cancel := context.WithTimeout(context.WithoutCancel(ctx), cfg.OperationWait)
-			err := processFastMessage(operationCtx, appender, updater, &fastMessage{
+			err := processFastMessage(operationCtx, registry, appender, updater, &fastMessage{
 				subject: msg.Subject,
 				data:    msg.Data,
 				ack: func() error {
@@ -235,7 +235,9 @@ type natsPullSubscription interface {
 	Fetch(int, ...nats.PullOpt) ([]*nats.Msg, error)
 }
 
-func processFastMessage(ctx context.Context, appender fastAppender, updater fastUpdater, msg *fastMessage) error {
+var fastWriterStageDurationBucketsMS = []float64{1, 5, 10, 25, 50, 100, 250, 500, 1000, 5000}
+
+func processFastMessage(ctx context.Context, registry *metrics.Registry, appender fastAppender, updater fastUpdater, msg *fastMessage) error {
 	var env envelope.FrameEnvelope
 	if err := json.Unmarshal(msg.data, &env); err != nil {
 		if msg.ack != nil {
@@ -243,14 +245,23 @@ func processFastMessage(ctx context.Context, appender fastAppender, updater fast
 		}
 		return nil
 	}
-	if err := appender.AppendAll(ctx, env); err != nil {
+	started := time.Now()
+	err := appender.AppendAll(ctx, env)
+	recordFastWriterStageDuration(registry, msg.subject, "tdengine", statusFromError(err), time.Since(started))
+	if err != nil {
 		return fmt.Errorf("tdengine append: %w", err)
 	}
-	if err := updater.FastUpdate(ctx, env); err != nil {
+	started = time.Now()
+	err = updater.FastUpdate(ctx, env)
+	recordFastWriterStageDuration(registry, msg.subject, "redis", statusFromError(err), time.Since(started))
+	if err != nil {
 		return fmt.Errorf("redis fast update: %w", err)
 	}
 	if msg.ack != nil {
-		if err := msg.ack(); err != nil {
+		started = time.Now()
+		err = msg.ack()
+		recordFastWriterStageDuration(registry, msg.subject, "ack", statusFromError(err), time.Since(started))
+		if err != nil {
 			return fmt.Errorf("nats ack: %w", err)
 		}
 	}
@@ -288,6 +299,24 @@ func addFastMetric(registry *metrics.Registry, subject string, status string) {
 	registry.IncCounter("vehicle_fast_writer_messages_total", metrics.Labels{"subject": subject, "status": status})
 }
 
+func recordFastWriterStageDuration(registry *metrics.Registry, subject string, stage string, status string, elapsed time.Duration) {
+	if registry == nil {
+		return
+	}
+	registry.ObserveHistogram("vehicle_fast_writer_stage_duration_ms_histogram", metrics.Labels{
+		"subject": subject,
+		"stage":   stage,
+		"status":  status,
+	}, fastWriterStageDurationBucketsMS, float64(elapsed.Milliseconds()))
+}
+
+func statusFromError(err error) string {
+	if err != nil {
+		return "error"
+	}
+	return "ok"
+}
+
 func env(key string, fallback string) string {
 	value := strings.TrimSpace(os.Getenv(key))
 	if value == "" {

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

@@ -3,9 +3,11 @@ package main
 import (
 	"context"
 	"errors"
+	"strings"
 	"testing"
 
 	"lingniu-vehicle-ingest/go/vehicle-gateway/internal/envelope"
+	"lingniu-vehicle-ingest/go/vehicle-gateway/internal/metrics"
 )
 
 func TestProcessFastMessageWritesTDengineAndRedisBeforeAck(t *testing.T) {
@@ -22,7 +24,7 @@ func TestProcessFastMessageWritesTDengineAndRedisBeforeAck(t *testing.T) {
 		return nil
 	}}
 
-	if err := processFastMessage(context.Background(), appender, updater, msg); err != nil {
+	if err := processFastMessage(context.Background(), nil, appender, updater, msg); err != nil {
 		t.Fatalf("processFastMessage() error = %v", err)
 	}
 	if appender.count != 1 || updater.count != 1 || ackCount != 1 {
@@ -44,7 +46,7 @@ func TestProcessFastMessageDoesNotAckWhenRedisFails(t *testing.T) {
 		return nil
 	}}
 
-	if err := processFastMessage(context.Background(), appender, updater, msg); err == nil {
+	if err := processFastMessage(context.Background(), nil, appender, updater, msg); err == nil {
 		t.Fatal("processFastMessage() error = nil, want redis failure")
 	}
 	if ackCount != 0 {
@@ -52,6 +54,32 @@ func TestProcessFastMessageDoesNotAckWhenRedisFails(t *testing.T) {
 	}
 }
 
+func TestProcessFastMessageRecordsStageDurationMetrics(t *testing.T) {
+	env := envelope.FrameEnvelope{Protocol: envelope.ProtocolJT808, VIN: "VIN001", EventID: "evt-3"}
+	payload, err := env.MarshalJSONBytes()
+	if err != nil {
+		t.Fatal(err)
+	}
+	registry := metrics.NewRegistry()
+	msg := &fastMessage{subject: "vehicle.raw.go.jt808.v1", data: payload, ack: func() error { return nil }}
+
+	if err := processFastMessage(context.Background(), registry, &recordingFastAppender{}, &recordingFastUpdater{}, msg); err != nil {
+		t.Fatalf("processFastMessage() error = %v", err)
+	}
+
+	text := registry.Render()
+	for _, want := range []string{
+		`vehicle_fast_writer_stage_duration_ms_histogram_bucket{le="+Inf",stage="tdengine",status="ok",subject="vehicle.raw.go.jt808.v1"} 1`,
+		`vehicle_fast_writer_stage_duration_ms_histogram_count{stage="tdengine",status="ok",subject="vehicle.raw.go.jt808.v1"} 1`,
+		`vehicle_fast_writer_stage_duration_ms_histogram_bucket{le="+Inf",stage="redis",status="ok",subject="vehicle.raw.go.jt808.v1"} 1`,
+		`vehicle_fast_writer_stage_duration_ms_histogram_bucket{le="+Inf",stage="ack",status="ok",subject="vehicle.raw.go.jt808.v1"} 1`,
+	} {
+		if !strings.Contains(text, want) {
+			t.Fatalf("fast writer stage metric missing %s:\n%s", want, text)
+		}
+	}
+}
+
 type recordingFastAppender struct {
 	count int
 	err   error