Modularize tracing executor and metrics rename (#6424)

* Tracing executor and metrics rename

* Appease clippy

* Merge branch 'unstable' into modularise-task-executor

validator_client/src/notifier.rs

@@ -1,6 +1,6 @@
 use crate::http_metrics;
 use crate::{DutiesService, ProductionValidatorClient};
-use lighthouse_metrics::set_gauge;
+use metrics::set_gauge;
 use slog::{debug, error, info, Logger};
 use slot_clock::SlotClock;
 use tokio::time::{sleep, Duration};