metrics for proposer and sync duties (#6028)

To avoid restarting with scheduled proposals or sync duties, which are
rare but have high rewards, add metrics to simpify tracking them.

beacon_chain/nimbus_beacon_node.nim

@@ -51,6 +51,12 @@ declareGauge ticks_delay,
 declareGauge next_action_wait,
   "Seconds until the next attestation will be sent"
 
+declareGauge next_proposal_wait,
+  "Seconds until the next proposal will be sent, or Inf if not known"
+
+declareGauge sync_committee_active,
+  "1 if there are current sync committee duties, 0 otherwise"
+
 declareCounter db_checkpoint_seconds,
   "Time spent checkpointing the database to clear the WAL file"
 
@@ -1466,19 +1472,21 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
     else:
       toGaugeValue(x)
 
+  let
+    syncCommitteeSlot = slot + 1
+    syncCommitteeEpoch = syncCommitteeSlot.epoch
+    inCurrentSyncCommittee =
+      not node.getCurrentSyncCommiteeSubnets(syncCommitteeEpoch).isZeros()
+
   template formatSyncCommitteeStatus(): string =
-    let
-      syncCommitteeSlot = slot + 1
-      slotsToNextSyncCommitteePeriod =
+    if inCurrentSyncCommittee:
+      "current"
+    elif not node.getNextSyncCommitteeSubnets(syncCommitteeEpoch).isZeros():
+      let slotsToNextSyncCommitteePeriod =
         SLOTS_PER_SYNC_COMMITTEE_PERIOD -
         since_sync_committee_period_start(syncCommitteeSlot)
-
-    # int64 conversion is safe
-    doAssert slotsToNextSyncCommitteePeriod <= SLOTS_PER_SYNC_COMMITTEE_PERIOD
-
-    if not node.getCurrentSyncCommiteeSubnets(syncCommitteeSlot.epoch).isZeros:
-      "current"
-    elif not node.getNextSyncCommitteeSubnets(syncCommitteeSlot.epoch).isZeros:
+      # int64 conversion is safe
+      doAssert slotsToNextSyncCommitteePeriod <= SLOTS_PER_SYNC_COMMITTEE_PERIOD
       "in " & toTimeLeftString(
         SECONDS_PER_SLOT.int64.seconds * slotsToNextSyncCommitteePeriod.int64)
     else:
@@ -1499,6 +1507,14 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
   if nextActionSlot != FAR_FUTURE_SLOT:
     next_action_wait.set(nextActionWaitTime.toFloatSeconds)
 
+  next_proposal_wait.set(
+    if nextProposalSlot != FAR_FUTURE_SLOT:
+      saturate(fromNow(node.beaconClock, nextProposalSlot)).toFloatSeconds()
+    else:
+      Inf)
+
+  sync_committee_active.set(if inCurrentSyncCommittee: 1 else: 0)
+
   let epoch = slot.epoch
   if epoch + 1 >= node.network.forkId.next_fork_epoch:
     # Update 1 epoch early to block non-fork-ready peers