Updates for the various Broker changes

NEWS

@@ -98,6 +98,26 @@ New Functionality
   options ``Broker::log_severity_level`` and ``Broker::log_stderr_severity_level`` have
   been introduced to control the which events to expose by default.
 
+- Broker's new per-peer send buffer backpressure handling, introduced in 7.1,
+  has received several updates. We've increased the default buffer sizes to 8192
+  messages for both peers and websockets, and switched the default overflow
+  handling policy to "drop_oldest", meaning that in a full buffer the oldest
+  message enqueued gets dropped to allow enqueuing a new one. Three additional
+  metrics are available to understand the health of each peering's buffer,
+  regardless of the overflow policy active. These are:
+
+  - zeek_broker_peer_buffer_levels: a gauge of the current buffer fill level,
+
+  - zeek_broker_peer_buffer_recent_max_levels: a gauge that tracks the maximum
+    buffer fill level seen over the last ``Broker::buffer_stats_reset_interval`.
+
+  - zeek_broker_peer_buffer_overflows_total: a counter that tracks the number
+    of times a given peering's send buffer has overflowed. For the "drop_oldest"
+    and "drop_newest" policies, this is the count of messages dropped.
+
+  Each of these is labeled with the current endpoint and the peer's, as provided
+  by the cluster topology.
+
 - New WebSocket functionality was added to Zeek's cluster component.
 
   Users of Broker's WebSocket interface should replace their ``Broker::listen_websocket()``
@@ -207,6 +227,9 @@ Changed Functionality
   connection attempts to ephemeral TCP client-side ports, which could clutter the Broker
   logs.
 
+- The connect and listen retry intervals of Broker and the Cluster framework
+  have all been reduced to one second, from previously 30s/60s.
+
 - The protocol confirmation for IRC was made more robust. It now checks for valid commands
   before confirming a connection as IRC.
 
@@ -229,6 +252,13 @@ Changed Functionality
   our switch to use the C-Ares library back in the 5.0 release, but we never removed the
   requirement from CMake.
 
+Removed Functionality
+---------------------
+
+- Broker's broker_buffered_messages metric has been removed, since the
+  backpressure handling introduced in 7.1 rendered it obsolete. Use the new
+  per-peering metrics described above instead.
+
 Deprecated Functionality
 ------------------------
 
@@ -238,6 +268,9 @@ Deprecated Functionality
 
 - ``Broker::listen_websocket()`` was deprecated in favor of ``Cluster::listen_websocket()`.
 
+- The ``Broker::congestion_queue_size`` tunable has had no effect since Zeek 5.0
+  and is slated for removal without replacement.
+
 Zeek 7.1.0
 ==========