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/src/cli.rs

@@ -232,6 +232,23 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(true),
         )
 
+        /*
+         * Monitoring 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),
+        )
+
         /*
          * Standard staking flags
          */

beacon_node/src/config.rs

@@ -136,6 +136,17 @@ pub fn get_config<E: EthSpec>(
         client_config.http_metrics.allow_origin = Some(allow_origin.to_string());
     }
 
+    /*
+     * Explorer metrics
+     */
+    if let Some(monitoring_endpoint) = cli_args.value_of("monitoring-endpoint") {
+        client_config.monitoring_api = Some(monitoring_api::Config {
+            db_path: None,
+            freezer_db_path: None,
+            monitoring_endpoint: monitoring_endpoint.to_string(),
+        });
+    }
+
     // Log a warning indicating an open HTTP server if it wasn't specified explicitly
     // (e.g. using the --staking flag).
     if cli_args.is_present("staking") {

beacon_node/src/lib.rs

@@ -63,14 +63,14 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
         let log = context.log().clone();
         let datadir = client_config.create_data_dir()?;
         let db_path = client_config.create_db_path()?;
-        let freezer_db_path_res = client_config.create_freezer_db_path();
+        let freezer_db_path = client_config.create_freezer_db_path()?;
         let executor = context.executor.clone();
 
         let builder = ClientBuilder::new(context.eth_spec_instance.clone())
             .runtime_context(context)
             .chain_spec(spec)
             .http_api_config(client_config.http_api.clone())
-            .disk_store(&datadir, &db_path, &freezer_db_path_res?, store_config)?;
+            .disk_store(&datadir, &db_path, &freezer_db_path, store_config)?;
 
         let builder = if let Some(slasher_config) = client_config.slasher.clone() {
             let slasher = Arc::new(
@@ -82,6 +82,14 @@ impl<E: EthSpec> ProductionBeaconNode<E> {
             builder
         };
 
+        let builder = if let Some(monitoring_config) = &mut client_config.monitoring_api {
+            monitoring_config.db_path = Some(db_path);
+            monitoring_config.freezer_db_path = Some(freezer_db_path);
+            builder.monitoring_client(monitoring_config)?
+        } else {
+            builder
+        };
+
         let builder = builder
             .beacon_chain_builder(client_genesis, client_config.clone())
             .await?;