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/block_lookups/tests.rs

@@ -1,5 +1,6 @@
 use std::sync::Arc;
 
+use crate::beacon_processor::BeaconProcessorSend;
 use crate::service::RequestId;
 use crate::sync::manager::RequestId as SyncId;
 use crate::NetworkMessage;
@@ -54,7 +55,7 @@ impl TestRig {
             SyncNetworkContext::new(
                 network_tx,
                 globals,
-                beacon_processor_tx,
+                BeaconProcessorSend(beacon_processor_tx),
                 log.new(slog::o!("component" => "network_context")),
             )
         };