Add specific log and metric for delayed blocks (#2308)

## Issue Addressed

NA

## Proposed Changes

- Adds a specific log and metric for when a block is enshrined as head with a delay that will caused bad attestations
    - We *technically* already expose this information, but it's a little tricky to determine during debugging. This makes it nice and explicit.
- Fixes a minor reporting bug with the validator monitor where it was expecting agg. attestations too early (at half-slot rather than two-thirds-slot).

## Additional Info

NA

common/slot_clock/src/lib.rs

@@ -75,4 +75,16 @@ pub trait SlotClock: Send + Sync + Sized + Clone {
         self.slot_of(self.now_duration()?.checked_sub(tolerance)?)
             .or_else(|| Some(self.genesis_slot()))
     }
+
+    /// Returns the delay between the start of the slot and when unaggregated attestations should be
+    /// produced.
+    fn unagg_attestation_production_delay(&self) -> Duration {
+        self.slot_duration() / 3
+    }
+
+    /// Returns the delay between the start of the slot and when aggregated attestations should be
+    /// produced.
+    fn agg_attestation_production_delay(&self) -> Duration {
+        self.slot_duration() * 2 / 3
+    }
 }