added debounce to log (#4269)

## Issue Addressed

[#4259](https://github.com/sigp/lighthouse/issues/4259)

## Proposed Changes

debounce spammy `Unable to send message to the beacon processor` log messages

## Additional Info

We could potentially debounce other logs that have the potential to be "spammy". 

After some feedback we decided to additionally add the following change:

create a newtype wrapper around `mpsc::Sender<BeaconWorkEvent<T>>`. When there is an error on the try_send method on the wrapper, we increase a counter metric with one label per work type.

beacon_node/network/src/metrics.rs

@@ -279,6 +279,12 @@ lazy_static! {
             "Gossipsub light_client_optimistic_update errors per error type",
             &["type"]
         );
+    pub static ref BEACON_PROCESSOR_SEND_ERROR_PER_WORK_TYPE: Result<IntCounterVec> =
+        try_create_int_counter_vec(
+            "beacon_processor_send_error_per_work_type",
+            "Total number of beacon processor send error per work type",
+            &["type"]
+        );
 
 
     /*