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

beacon_node/beacon_chain/src/metrics.rs

@@ -629,6 +629,11 @@ lazy_static! {
         "beacon_block_head_slot_start_delay_time",
         "Duration between the start of the blocks slot and the current time when it was as head.",
     );
+    pub static ref BEACON_BLOCK_HEAD_SLOT_START_DELAY_EXCEEDED_TOTAL: Result<IntCounter> = try_create_int_counter(
+        "beacon_block_head_slot_start_delay_exceeded_total",
+        "Triggered when the duration between the start of the blocks slot and the current time \
+        will result in failed attestations.",
+    );
 
     /*
      * General block metrics