Sync metrics (#1975)

## Issue Addressed
- Add metrics to keep track of peer counts by sync type
- Add metric to keep track of the number of syncing chains in range

## Proposed Changes
Plugin to the network metrics update interval and update too the counts for peers wrt to their sync status with us

## Additional Info
For the peer counts
- By the way it is implemented the numbers won't always match to the total peer count in the `libp2p` metric.
- Updating the gauge with every change is messy because it requires to be updated on connection (in the `eth2_libp2p` crate, while metrics are defined in the `network` crate) on Goodbye sent (for an `IrrelevantPeer`) either in the `beacon_processor` or the `peer_manager`, and on disconnection. Since this is not a critical metric I think counting once every second is enough. If you think more accuracy is needed we can do it too, but it would be harder to maintain)

ATM those look like this
![image](https://user-images.githubusercontent.com/26765164/100275387-22137b00-2f60-11eb-93b9-94b0f265240c.png)

beacon_node/network/src/service.rs

@@ -14,6 +14,7 @@ use eth2_libp2p::{
     types::GossipKind, BehaviourEvent, GossipTopic, MessageId, NetworkGlobals, PeerId, TopicHash,
 };
 use eth2_libp2p::{MessageAcceptance, Service as LibP2PService};
+use fnv::FnvHashMap;
 use futures::prelude::*;
 use slog::{debug, error, info, o, trace, warn};
 use std::{collections::HashMap, net::SocketAddr, sync::Arc, time::Duration};
@@ -277,6 +278,9 @@ fn spawn_service<T: BeaconChainTypes>(
                         &service.libp2p.swarm.gs(),
                         &service.network_globals,
                     );
+                    // update sync metrics
+                    update_sync_metrics(&service.network_globals);
+
                 }
                 _ = service.gossipsub_parameter_update.next() => {
                     if let Ok(slot) = service.beacon_chain.slot() {
@@ -932,3 +936,29 @@ fn update_gossip_metrics<T: EthSpec>(
         }
     }
 }
+
+fn update_sync_metrics<T: EthSpec>(network_globals: &Arc<NetworkGlobals<T>>) {
+    // reset the counts
+    if metrics::PEERS_PER_SYNC_TYPE
+        .as_ref()
+        .map(|metric| metric.reset())
+        .is_err()
+    {
+        return;
+    };
+
+    // count per sync status, the number of connected peers
+    let mut peers_per_sync_type = FnvHashMap::default();
+    for sync_type in network_globals
+        .peers
+        .read()
+        .connected_peers()
+        .map(|(_peer_id, info)| info.sync_status.as_str())
+    {
+        *peers_per_sync_type.entry(sync_type).or_default() += 1;
+    }
+
+    for (sync_type, peer_count) in peers_per_sync_type {
+        metrics::set_gauge_entry(&metrics::PEERS_PER_SYNC_TYPE, &[sync_type], peer_count);
+    }
+}