Configurable monitoring endpoint frequency (#3530)

## Issue Addressed

Closes #3514

## Proposed Changes

- Change default monitoring endpoint frequency to 120 seconds to fit with 30k requests/month limit.
- Allow configuration of the monitoring endpoint frequency using `--monitoring-endpoint-frequency N` where `N` is a value in seconds.

validator_client/src/cli.rs

@@ -236,6 +236,15 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 and never provide an untrusted URL.")
                 .takes_value(true),
         )
+        .arg(
+            Arg::with_name("monitoring-endpoint-period")
+                .long("monitoring-endpoint-period")
+                .value_name("SECONDS")
+                .help("Defines how many seconds to wait between each message sent to \
+                       the monitoring-endpoint. Default: 60s")
+                .requires("monitoring-endpoint")
+                .takes_value(true),
+        )
         .arg(
             Arg::with_name("enable-doppelganger-protection")
                 .long("enable-doppelganger-protection")

validator_client/src/config.rs

@@ -296,9 +296,12 @@ impl Config {
          * Explorer metrics
          */
         if let Some(monitoring_endpoint) = cli_args.value_of("monitoring-endpoint") {
+            let update_period_secs =
+                clap_utils::parse_optional(cli_args, "monitoring-endpoint-period")?;
             config.monitoring_api = Some(monitoring_api::Config {
                 db_path: None,
                 freezer_db_path: None,
+                update_period_secs,
                 monitoring_endpoint: monitoring_endpoint.to_string(),
             });
         }