feat: add metric for slot of next scheduled attestation duty

packages/validator/src/metrics.ts

@@ -206,6 +206,11 @@ export function getMetrics(register: MetricsRegister, gitData: LodestarGitData)
       help: "Total count of instances the attester duties dependant root changed",
     }),
 
+    attesterDutiesNextSlot: register.gauge({
+      name: "vc_attestation_duty_slot",
+      help: "Slot of next scheduled attestation duty",
+    }),
+
     // BlockProposingService
 
     blocksProduced: register.gauge({

packages/validator/src/services/attestationDuties.ts

@@ -63,12 +63,32 @@ export class AttestationDutiesService {
 
     if (metrics) {
       metrics.attesterDutiesCount.addCollect(() => {
+        const currentSlot = this.clock.getCurrentSlot();
         let duties = 0;
-        for (const attDutiesAtEpoch of this.dutiesByIndexByEpoch.values()) {
+        let nextDutySlot = null;
+        for (const [epoch, attDutiesAtEpoch] of this.dutiesByIndexByEpoch) {
           duties += attDutiesAtEpoch.dutiesByIndex.size;
+
+          // Historical epochs can be skipped when determining next duty slot
+          if (epoch < this.clock.currentEpoch) continue;
+
+          // There is no need to check every single duty at a certain amount
+          // of validators, next duty slot will almost always be the next slot
+          if (attDutiesAtEpoch.dutiesByIndex.size >= 64) {
+            nextDutySlot = currentSlot + 1;
+            continue;
+          }
+
+          for (const {duty} of attDutiesAtEpoch.dutiesByIndex.values()) {
+            // Set next duty slot to the closest future slot found in all duties
+            if (duty.slot > currentSlot && (nextDutySlot === null || duty.slot < nextDutySlot)) {
+              nextDutySlot = duty.slot;
+            }
+          }
         }
         metrics.attesterDutiesCount.set(duties);
         metrics.attesterDutiesEpochCount.set(this.dutiesByIndexByEpoch.size);
+        if (nextDutySlot !== null) metrics.attesterDutiesNextSlot.set(nextDutySlot);
       });
     }
   }