core: Fix noisy logs caused by closing the `/feed` browser

[lexnv]

Multiple warning messages originated from the telemetry-core' regarding flushing the WS connection.

WARN  [telemetry_core] Closing feed websocket due to error flushing data: connection closed

These errors can happen when the feed components (browsers) are closing the connection unexpectedly. This behavior can be reproduced with the following patch, added between sending and flushing the data:

--- a/backend/telemetry_core/src/main.rs
+++ b/backend/telemetry_core/src/main.rs
@@ -474,6 +474,11 @@ where
                     Ok(_) => {}
                 }
             }
+
+            log::info!("Sent bytes, will flush the feed next. Close the browser now!");
+            tokio::time::sleep(Duration::from_secs(5)).await;
+            log::info!("Finished sleeping");
+
             match tokio::time::timeout_at(message_send_deadline, ws_send.flush()).await {

To mitigate noisy logs, change the log level to debug for the cases of abruptly closing the socket.