Fix proxy_io_bytes_per_client metric: use branch_id identifier properly. (#4084)

It fixes the miscalculation of the metric for projects that use multiple
branches for the same endpoint.
We were under billing users with such projects. So we need to
communicate the change in Release Notes.

proxy/src/metrics.rs

@@ -95,7 +95,7 @@ fn gather_proxy_io_bytes_per_client() -> Vec<(Ids, (u64, DateTime<Utc>))> {
                     current_metrics.push((
                         Ids {
                             endpoint_id: endpoint_id.to_string(),
-                            branch_id: "".to_string(),
+                            branch_id: branch_id.to_string(),
                         },
                         (value, Utc::now()),
                     ));