Add metrics to track beacon processor queue times (#7808)

This PR adds a created_timestamp to the beacon processor send channel. When work items are sent through that channel `try_send` will forward the work event along with the current timestamp to the beacon processor. When the work event is completed the `Drop` impl for `SendOnDrop` will track the time it took from work event creation to its completion. Previously we only had data on how long a work event took to process, but didn't have data on how long it sat in the queue + how long it took to process.

beacon_node/beacon_processor/src/metrics.rs

@@ -128,3 +128,10 @@ pub static BEACON_PROCESSOR_SEND_ERROR_PER_WORK_TYPE: LazyLock<Result<IntCounter
             &["type"],
         )
     });
+pub static BEACON_PROCESSOR_QUEUE_TIME: LazyLock<Result<HistogramVec>> = LazyLock::new(|| {
+    try_create_histogram_vec(
+        "beacon_processor_queue_time",
+        "The delay between when a work event was queued in the beacon processor and when it was popped from the queue",
+        &["work_type"]
+    )
+});