add metric for nextActionWait (#2399)

* add metric for nextActionWait

* use toFloatSeconds

beacon_chain/gossip_processing/consensus_manager.nim

@@ -9,7 +9,7 @@
 
 import
   chronicles, chronos,
-  ../spec/[crypto, datatypes, digest],
+  ../spec/[crypto, datatypes],
   ../consensus_object_pools/[blockchain_dag, attestation_pool]
 
 # TODO: Move to "consensus_object_pools" folder

beacon_chain/gossip_processing/eth2_processor.nim

@@ -8,7 +8,7 @@
 {.push raises: [Defect].}
 
 import
-  std/[math, tables],
+  std/tables,
   stew/results,
   chronicles, chronos, metrics,
   ../spec/[crypto, datatypes, digest],

beacon_chain/nimbus_beacon_node.nim

@@ -67,6 +67,9 @@ declareGauge finalization_delay,
 declareGauge ticks_delay,
   "How long does to take to run the onSecond loop"
 
+declareGauge next_action_wait,
+  "Seconds until the next attestation will be sent"
+
 logScope: topics = "beacnde"
 
 func enrForkIdFromState(state: BeaconState): ENRForkID =
@@ -928,7 +931,8 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
     horizonSlot = compute_start_slot_at_epoch(
       node.attestationSubnets.lastCalculatedAttestationEpoch + 1) - 1
     nextAttestationSlot = node.getNextAttestation(slot)
-    nextActionWait = saturate(fromNow(node.beaconClock, nextAttestationSlot))
+    nextActionWaitTime =
+      saturate(fromNow(node.beaconClock, nextAttestationSlot))
     horizonDistance = saturate(fromNow(node.beaconClock, horizonSlot))
 
   info "Slot end",
@@ -944,9 +948,12 @@ proc onSlotEnd(node: BeaconNode, slot: Slot) {.async.} =
       if nextAttestationSlot == FAR_FUTURE_SLOT:
         "n/a"
       else:
-        shortLog(nextActionWait),
+        shortLog(nextActionWaitTime),
     lookaheadTime = shortLog(horizonDistance)
 
+  if nextAttestationSlot != FAR_FUTURE_SLOT:
+    next_action_wait.set(nextActionWaitTime.toFloatSeconds)
+
   node.updateGossipStatus(slot)
 
 proc onSlotStart(