Add more metrics for tracking sync messages (#4308)

## Issue Addressed

NA

## Proposed Changes

Adds metrics to track validators that are submitting equivocating (but not slashable) sync messages. This follows on from some research we've been doing in a separate fork of LH.

## Additional Info

@jimmygchen and @michaelsproul have already run their eyes over this so it should be easy to get into v4.2.0, IMO.

beacon_node/beacon_chain/src/metrics.rs

@@ -874,6 +874,14 @@ lazy_static! {
         "beacon_sync_committee_message_gossip_verification_seconds",
         "Full runtime of sync contribution gossip verification"
     );
+    pub static ref SYNC_MESSAGE_EQUIVOCATIONS: Result<IntCounter> = try_create_int_counter(
+        "sync_message_equivocations_total",
+        "Number of sync messages with the same validator index for different blocks"
+    );
+    pub static ref SYNC_MESSAGE_EQUIVOCATIONS_TO_HEAD: Result<IntCounter> = try_create_int_counter(
+        "sync_message_equivocations_to_head_total",
+        "Number of sync message which conflict with a previous message but elect the head"
+    );
 
     /*
      * Sync Committee Contribution Verification