Metrics and DEBG log for late gossip blocks (#2533)

## Issue Addressed

Which issue # does this PR address?

## Proposed Changes

- Add a counter metric to log when a block is received late from gossip.
- Also push a `DEBG` log for the above condition.
- Use Debug (`?`) instead of Display (`%`) for a bunch of logs in the beacon processor, so we don't have to deal with concatenated block roots.
- Add new ERRO and CRIT to HTTP API to alert users when they're publishing late blocks.

## Additional Info

NA

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -2932,6 +2932,7 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             .beacon_state
             .previous_epoch()
             .start_slot(T::EthSpec::slots_per_epoch());
+        let head_proposer_index = new_head.beacon_block.message().proposer_index();
 
         // Update the snapshot that stores the head of the chain at the time it received the
         // block.
@@ -2962,9 +2963,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             debug!(
                 self.log,
                 "Delayed head block";
-                "delay" => ?block_delay,
-                "root" => ?beacon_block_root,
+                "block_root" => ?beacon_block_root,
+                "proposer_index" => head_proposer_index,
                 "slot" => head_slot,
+                "block_delay" => ?block_delay,
             );
         }