feat(metrics): 投递失败与死信可观测指标，修 flushSchd 首次失败原因丢失（ACM2-98）

- PipelineCounters 按 target 累计发送失败；Dispatcher.retryOrDead 记账（OPS-2）
- 新增 deadByTarget 查询与 DeliveryDeadSnapshotProvider（TTL 缓存），死信存量按 target 暴露
- /metrics 注册 send_failures.total{target} 与 dead{target}，登记进 reference.md「指标与健康」
- flushSchd 失败改记真实异常原因，不再退回固定 send-failed

src/test/kotlin/com/gzzn/omms/msgexchange/delivery/DispatcherTickTest.kt

@@ -8,6 +8,7 @@ import com.gzzn.omms.msgexchange.domain.EventType
 import com.gzzn.omms.msgexchange.domain.MsgEvent
 import com.gzzn.omms.msgexchange.domain.Targets
 import com.gzzn.omms.msgexchange.infra.persistence.MsgEventRepository
+import com.gzzn.omms.msgexchange.infra.metrics.PipelineCounters
 import com.gzzn.omms.msgexchange.infra.retry.FailureScheduler
 import com.gzzn.omms.msgexchange.infra.stub.StubDeliveryPort
 import com.gzzn.omms.msgexchange.infra.stub.StubMsgEvents
@@ -28,8 +29,12 @@ class DispatcherTickTest {
 
     private val clock = MutableClock(MutableClock.BASE)
 
-    private fun dispatcher(repo: MsgEventRepository, port: DeliveryPort, p: PipelineProps = PipelineProps()): Dispatcher =
-        Dispatcher(repo, port, p, FailureScheduler(p, clock))
+    private fun dispatcher(
+        repo: MsgEventRepository,
+        port: DeliveryPort,
+        p: PipelineProps = PipelineProps(),
+        counters: PipelineCounters = PipelineCounters(),
+    ): Dispatcher = Dispatcher(repo, port, p, FailureScheduler(p, clock), counters)
 
     private fun ev(id: Long, target: String, key: String, payload: String, version: Long = 0) =
         MsgEvent(eventId = id, target = target, partitionKey = key, stateVersion = version, payloadJson = payload, createdAt = MutableClock.BASE)
@@ -123,11 +128,14 @@ class DispatcherTickTest {
     fun `schd send failure schedules backoff and goes DEAD at limit`() {
         val repo = StubMsgEvents()
         val p = PipelineProps()
-        val d = dispatcher(repo, FailingSchdPort(), p)
+        val counters = PipelineCounters()
+        val d = dispatcher(repo, FailingSchdPort(), p, counters)
         repo.insertAll(listOf(ev(1, Targets.KAFKA_SCHD, "F1", """{"v":"1"}""", 1)))
 
         d.flushSchd()
         assertEquals(1, repo.rows[1]!!.attempts)
+        // OPS-2：发送失败按 target 计数，且失败原因不是固定字面量
+        assertEquals(1L, counters.sendFailedCount(Targets.KAFKA_SCHD))
 
         repeat(p.pipeline.maxAttempts - 1) {
             clock.advance(p.pipeline.backoffFor(repo.rows[1]!!.attempts) + 1)
@@ -135,6 +143,7 @@ class DispatcherTickTest {
         }
         val final = repo.rows[1]!!
         assertTrue(final.attempts >= p.pipeline.maxAttempts || final.state.name == "DEAD")
+        assertEquals(0L, counters.sendFailedCount(Targets.KAFKA_MSG))   // msg 目标不受 schd 失败影响
     }
 
     @Test

src/test/kotlin/com/gzzn/omms/msgexchange/infra/metrics/PipelineMetricsTest.kt

@@ -1,6 +1,10 @@
 package com.gzzn.omms.msgexchange.infra.metrics
 
+import com.gzzn.omms.msgexchange.domain.ErrorClass
+import com.gzzn.omms.msgexchange.domain.MsgEvent
 import com.gzzn.omms.msgexchange.domain.ProcStatus
+import com.gzzn.omms.msgexchange.domain.Targets
+import com.gzzn.omms.msgexchange.infra.stub.StubMsgEvents
 import com.gzzn.omms.msgexchange.infra.stub.StubProcState
 import io.micrometer.core.instrument.MeterRegistry
 import io.micronaut.context.ApplicationContext
@@ -74,6 +78,34 @@ class PipelineMetricsTest {
         assertEquals(vipfBefore + 1.0, gauge("msgx.pipeline.codec.vipf_seen.total"), 0.001)
     }
 
+    @Test
+    fun `delivery send-failure and dead-letter gauges are registered per target`() {
+        val counters = ctx.getBean(PipelineCounters::class.java)
+        val events = ctx.getBean(StubMsgEvents::class.java)
+        val failuresBefore = taggedGauge("msgx.pipeline.delivery.send_failures.total", Targets.KAFKA_SCHD)
+
+        counters.sendFailedAdd(Targets.KAFKA_SCHD)
+        val ids = events.insertAll(
+            listOf(
+                MsgEvent(target = Targets.KAFKA_MSG, partitionKey = "F1", stateVersion = 1, payloadJson = "{}", createdAt = Instant.now()),
+            ),
+        )
+        events.markDead(ids.single(), ErrorClass.EXHAUSTED, "boom", attempts = 5)
+
+        assertEquals(
+            failuresBefore + 1.0,
+            taggedGauge("msgx.pipeline.delivery.send_failures.total", Targets.KAFKA_SCHD),
+            0.001,
+        )
+        assertEquals(1.0, taggedGauge("msgx.pipeline.delivery.dead", Targets.KAFKA_MSG), 0.001)
+        assertEquals(0.0, taggedGauge("msgx.pipeline.delivery.dead", Targets.KAFKA_SCHD), 0.001)
+    }
+
     private fun gauge(name: String): Double =
         requireNotNull(registry.find(name).gauge()) { "gauge not registered: $name" }.value()
+
+    private fun taggedGauge(name: String, target: String): Double =
+        requireNotNull(registry.find(name).tag("target", target).gauge()) {
+            "gauge not registered: $name(target=$target)"
+        }.value()
 }