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

beacon_node/beacon_chain/src/metrics.rs

@@ -54,6 +54,10 @@ lazy_static! {
         "beacon_block_processing_attestation_observation_seconds",
         "Time spent hashing and remembering all the attestations in the block"
     );
+    pub static ref BLOCK_SYNC_AGGREGATE_SET_BITS: Result<IntGauge> = try_create_int_gauge(
+        "block_sync_aggregate_set_bits",
+        "The number of true bits in the last sync aggregate in a block"
+    );
 
     /*
      * Block Production