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/sync/range_sync/range.rs

@@ -375,7 +375,7 @@ mod tests {
     use crate::NetworkMessage;
 
     use super::*;
-    use crate::beacon_processor::WorkEvent as BeaconWorkEvent;
+    use crate::beacon_processor::{BeaconProcessorSend, WorkEvent as BeaconWorkEvent};
     use beacon_chain::builder::Witness;
     use beacon_chain::eth1_chain::CachingEth1Backend;
     use beacon_chain::parking_lot::RwLock;
@@ -603,7 +603,7 @@ mod tests {
         let cx = SyncNetworkContext::new(
             network_tx,
             globals.clone(),
-            beacon_processor_tx,
+            BeaconProcessorSend(beacon_processor_tx),
             log.new(o!("component" => "network_context")),
         );
         let test_rig = TestRig {