feat(go): expose fast writer nats consumer 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_nats_consumer_ack_pending` | 稳态为 0 |
+| NATS fast writer | `vehicle_fast_writer_nats_consumer_pending` | burst 后下降 |
 | 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 |

docs/ops/go-service-observability.md

@@ -47,6 +47,9 @@ 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_nats_consumer_pending` | JetStream messages pending for the fast writer durable consumer. |
+| `vehicle_fast_writer_nats_consumer_ack_pending` | JetStream messages delivered to fast writer but not yet acked. |
+| `vehicle_fast_writer_nats_consumer_waiting` | Pull requests waiting on the fast writer durable consumer. |
 | `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`. |

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

@@ -426,11 +426,14 @@ vehicle_fast_writer_stage_duration_ms_histogram_sum{subject,stage,status}
 同时新增 batch pending gauge：
 
 ```text
+vehicle_fast_writer_nats_consumer_pending{stream,consumer}
+vehicle_fast_writer_nats_consumer_ack_pending{stream,consumer}
+vehicle_fast_writer_nats_consumer_waiting{stream,consumer}
 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。
+`vehicle_fast_writer_nats_consumer_*` 表示 JetStream durable consumer 层面的 backlog/ack-pending；`vehicle_fast_writer_batch_pending_*` 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 NATS 拉取、TDengine、Redis 还是 ack。
 
 ### NATS Kafka bridge batch 指标
 

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_nats_consumer_ack_pending` | 连续 2 分钟 `> 0` | 消息已投递给 fast-writer，但 TDengine/Redis 写入后未完成 ack。 |
+| `vehicle_fast_writer_nats_consumer_pending` | 持续增长且 `> 10000` | fast-writer 消费 NATS 的速度跟不上入口写入速度。 |
 | `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。 |