Monitoring service api (#2251)

## Issue Addressed

N/A

## Proposed Changes

Adds a client side api for collecting system and process metrics and pushing it to a monitoring service.

beacon_node/client/src/metrics.rs

@@ -6,4 +6,14 @@ lazy_static! {
         "sync_slots_per_second",
         "The number of blocks being imported per second"
     );
+
+    pub static ref IS_SYNCED: Result<IntGauge> = try_create_int_gauge(
+        "sync_eth2_synced",
+        "Metric to check if the beacon chain is synced to head. 0 if not synced and non-zero if synced"
+    );
+
+    pub static ref NOTIFIER_HEAD_SLOT: Result<IntGauge> = try_create_int_gauge(
+        "notifier_head_slot",
+        "The head slot sourced from the beacon chain notifier"
+    );
 }