Fix unaggregated delay metric (#8366)

while working on this #7892 @michaelsproul pointed it might be a good metric to measure the delay from start of the slot instead of the current `slot_duration / 3`, since the attestations duties start before the `1/3rd` mark now with the change in the link PR.


Co-Authored-By: hopinheimer <knmanas6@gmail.com>

Co-Authored-By: hopinheimer <48147533+hopinheimer@users.noreply.github.com>

beacon_node/beacon_chain/src/validator_monitor.rs

@@ -1214,7 +1214,7 @@ impl<E: EthSpec> ValidatorMonitor<E> {
         let delay = get_message_delay_ms(
             seen_timestamp,
             data.slot,
-            slot_clock.unagg_attestation_production_delay(),
+            Duration::from_secs(0),
             slot_clock,
         );