UID2-7063: Emit salts_rotated_last_cycle Prometheus gauge

src/main/java/com/uid2/admin/Main.java

@@ -14,6 +14,7 @@
 import com.uid2.admin.legacy.RotatingLegacyClientKeyProvider;
 import com.uid2.admin.managers.KeysetManager;
 import com.uid2.admin.monitoring.DataStoreMetrics;
+import com.uid2.admin.monitoring.SaltRotationMetrics;
 import com.uid2.admin.salt.SaltRotation;
 import com.uid2.admin.secret.*;
 import com.uid2.admin.store.*;
@@ -329,6 +330,7 @@ public void run() {
             DataStoreMetrics.addDataStoreMetrics("service_link", serviceLinkProvider);
             DataStoreMetrics.addDataStoreServiceLinkEntryCount("snowflake", serviceLinkProvider, serviceProvider);
 
+            SaltRotationMetrics.register(Metrics.globalRegistry);
 
             ReplaceSharingTypesWithSitesJob replaceSharingTypesWithSitesJob = new ReplaceSharingTypesWithSitesJob(config, writeLock, adminKeysetProvider, keysetProvider, keysetStoreWriter, siteProvider);
             jobDispatcher.enqueue(replaceSharingTypesWithSitesJob);

src/main/java/com/uid2/admin/monitoring/SaltRotationMetrics.java

@@ -0,0 +1,29 @@
+package com.uid2.admin.monitoring;
+
+import io.micrometer.core.instrument.Gauge;
+import io.micrometer.core.instrument.MeterRegistry;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+public final class SaltRotationMetrics {
+    private static final AtomicLong lastRotatedSaltCount = new AtomicLong(-1);
+
+    public static void register(MeterRegistry registry) {
+        // Reports NaN until the first rotation completes, so the alert does not fire on cold start.
+        Gauge.builder("uid2_salts_rotated_last_cycle", lastRotatedSaltCount, SaltRotationMetrics::asGaugeValue)
+                .description("Number of salts rotated in the most recent successful salt rotation cycle")
+                .strongReference(true)
+                .register(registry);
+    }
+
+    public static void recordRotated(int count) {
+        lastRotatedSaltCount.set(count);
+    }
+
+    private static double asGaugeValue(AtomicLong ref) {
+        long value = ref.get();
+        return value < 0 ? Double.NaN : (double) value;
+    }
+
+    private SaltRotationMetrics() {}
+}

src/main/java/com/uid2/admin/salt/SaltRotation.java

@@ -1,6 +1,7 @@
 package com.uid2.admin.salt;
 
 import com.uid2.admin.AdminConst;
+import com.uid2.admin.monitoring.SaltRotationMetrics;
 import com.uid2.shared.model.SaltEntry;
 import com.uid2.shared.secret.IKeyGenerator;
 
@@ -63,6 +64,7 @@ public Result rotateSalts(
         logSaltAges("rotated-salts", targetDate, saltsToRotate);
         logSaltAges("total-salts", targetDate, Arrays.asList(postRotationSalts));
         logBucketFormatCount(targetDate, postRotationSalts);
+        SaltRotationMetrics.recordRotated(saltsToRotate.size());
 
         var nextSnapshot = new SaltSnapshot(
                 nextEffective,

src/test/java/com/uid2/admin/salt/SaltRotationTest.java

@@ -3,10 +3,12 @@
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import ch.qos.logback.core.read.ListAppender;
 import com.uid2.admin.AdminConst;
+import com.uid2.admin.monitoring.SaltRotationMetrics;
 import com.uid2.admin.salt.helper.SaltBuilder;
 import com.uid2.admin.salt.helper.SaltSnapshotBuilder;
 import com.uid2.shared.model.SaltEntry;
 import com.uid2.shared.secret.IKeyGenerator;
+import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
 import io.vertx.core.json.JsonObject;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -440,6 +442,61 @@ private int countEntriesWithLastUpdated(SaltEntry[] entries, Instant lastUpdated
         return (int) Arrays.stream(entries).filter(e -> e.lastUpdated() == lastUpdated.toEpochMilli()).count();
     }
 
+    @Test
+    void testRotateSaltsRecordsLastCycleMetric() throws Exception {
+        var registry = new SimpleMeterRegistry();
+        SaltRotationMetrics.register(registry);
+        SaltRotationMetrics.recordRotated(-1);
+
+        final Duration[] minAges = {
+                Duration.ofDays(1),
+                Duration.ofDays(2),
+        };
+        var lastSnapshot = SaltSnapshotBuilder.start()
+                .entries(10, daysEarlier(10), targetDate())
+                .build();
+
+        var result = saltRotation.rotateSalts(lastSnapshot, minAges, 0.2, targetDate());
+        assertTrue(result.hasSnapshot());
+
+        var rotatedCount = countEntriesWithLastUpdated(result.getSnapshot().getAllRotatingSalts(), result.getSnapshot().getEffective());
+        var gauge = registry.find("uid2_salts_rotated_last_cycle").gauge();
+        assertThat(gauge).isNotNull();
+        assertThat(gauge.value()).isEqualTo((double) rotatedCount);
+    }
+
+    @Test
+    void testRotateSaltsNoSnapshotLeavesMetricUnchanged() throws Exception {
+        var registry = new SimpleMeterRegistry();
+        SaltRotationMetrics.register(registry);
+        SaltRotationMetrics.recordRotated(42);
+
+        final Duration[] minAges = {
+                Duration.ofDays(1),
+                Duration.ofDays(2),
+        };
+        var lastSnapshot = SaltSnapshotBuilder.start()
+                .entries(10, targetDate(), targetDate())
+                .build();
+
+        var result = saltRotation.rotateSalts(lastSnapshot, minAges, 0.2, targetDate());
+        assertFalse(result.hasSnapshot());
+
+        var gauge = registry.find("uid2_salts_rotated_last_cycle").gauge();
+        assertThat(gauge.value()).isEqualTo(42.0);
+    }
+
+    @Test
+    void testSaltRotationMetricReportsNaNBeforeFirstRotation() {
+        var registry = new SimpleMeterRegistry();
+        SaltRotationMetrics.register(registry);
+        SaltRotationMetrics.recordRotated(-1);
+
+        var gauge = registry.find("uid2_salts_rotated_last_cycle").gauge();
+        assertThat(gauge).isNotNull();
+        assertThat(Double.isNaN(gauge.value())).isTrue();
+    }
+
     @Test
     void testRotateSaltsZeroDoesntRotateSaltsButUpdatesRefreshFrom() throws Exception {
         var lastSnapshot = SaltSnapshotBuilder.start()