Add interop metrics (#2645)

## Issue Addressed

Resolves: #2644

## Proposed Changes

- Adds mandatory metrics mentioned here: https://github.com/ethereum/beacon-metrics/blob/master/metrics.md#interop-metrics

## Additional Info

Couldn't figure out how to alias metrics, so I created them all as new gauges/counters.

Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/eth2_libp2p/src/metrics.rs

@@ -9,6 +9,8 @@ lazy_static! {
         "libp2p_peer_connected_peers_total",
         "Count of libp2p peers currently connected"
     );
+    pub static ref PEERS_CONNECTED_INTEROP: Result<IntGauge> =
+        try_create_int_gauge("libp2p_peers", "Count of libp2p peers currently connected");
     pub static ref PEER_CONNECT_EVENT_COUNT: Result<IntCounter> = try_create_int_counter(
         "libp2p_peer_connect_event_total",
         "Count of libp2p peer connect events (not the current number of connected peers)"