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.

validator_client/src/cli.rs

@@ -181,4 +181,19 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     address of this server (e.g., http://localhost:5064).")
                 .takes_value(true),
         )
+        /*
+         * Explorer metrics
+         */
+         .arg(
+            Arg::with_name("monitoring-endpoint")
+                .long("monitoring-endpoint")
+                .value_name("ADDRESS")
+                .help("Enables the monitoring service for sending system metrics to a remote endpoint. \
+                This can be used to monitor your setup on certain services (e.g. beaconcha.in). \
+                This flag sets the endpoint where the beacon node metrics will be sent. \
+                Note: This will send information to a remote sever which may identify and associate your \
+                validators, IP address and other personal information. Always use a HTTPS connection \
+                and never provide an untrusted URL.")
+                .takes_value(true),
+        )
 }