Metrics for sync aggregate fullness (#2439)

## Issue Addressed

NA

## Proposed Changes

Adds a metric to see how many set bits are in the sync aggregate for each beacon block being imported.

## Additional Info

NA

consensus/types/src/beacon_block_body.rs

@@ -45,6 +45,16 @@ pub struct BeaconBlockBody<T: EthSpec> {
     pub sync_aggregate: SyncAggregate<T>,
 }
 
+impl<'a, T: EthSpec> BeaconBlockBodyRef<'a, T> {
+    /// Access the sync aggregate from the block's body, if one exists.
+    pub fn sync_aggregate(self) -> Option<&'a SyncAggregate<T>> {
+        match self {
+            BeaconBlockBodyRef::Base(_) => None,
+            BeaconBlockBodyRef::Altair(inner) => Some(&inner.sync_aggregate),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     mod base {

consensus/types/src/sync_aggregate.rs

@@ -33,4 +33,9 @@ impl<T: EthSpec> SyncAggregate<T> {
             sync_committee_signature: AggregateSignature::empty(),
         }
     }
+
+    /// Returns how many bits are `true` in `self.sync_committee_bits`.
+    pub fn num_set_bits(&self) -> usize {
+        self.sync_committee_bits.num_set_bits()
+    }
 }