proxy: report redis broken message metric (#7021)

## Problem

Not really a problem. Improving visibility around redis communication.

## Summary of changes

Added metric on the number of broken messages.

proxy/src/metrics.rs

@@ -303,3 +303,12 @@ pub static ENDPOINT_ERRORS_BY_KIND: Lazy<HyperLogLogVec<32>> = Lazy::new(|| {
     )
     .unwrap()
 });
+
+pub static REDIS_BROKEN_MESSAGES: Lazy<IntCounterVec> = Lazy::new(|| {
+    register_int_counter_vec!(
+        "proxy_redis_errors_total",
+        "Number of errors by a given classification",
+        &["channel"],
+    )
+    .unwrap()
+});

proxy/src/redis/notifications.rs

@@ -10,6 +10,7 @@ use crate::{
     cache::project_info::ProjectInfoCache,
     cancellation::{CancelMap, CancellationHandler, NotificationsCancellationHandler},
     intern::{ProjectIdInt, RoleNameInt},
+    metrics::REDIS_BROKEN_MESSAGES,
 };
 
 const CPLANE_CHANNEL_NAME: &str = "neondb-proxy-ws-updates";
@@ -115,6 +116,9 @@ impl<
         let msg: Notification = match serde_json::from_str(&payload) {
             Ok(msg) => msg,
             Err(e) => {
+                REDIS_BROKEN_MESSAGES
+                    .with_label_values(&[msg.get_channel_name()])
+                    .inc();
                 tracing::error!("broken message: {e}");
                 return Ok(());
             }