Add enableBrokerTopicListWatcher config and explained that topic list watch is not supported.

Author: coderzc

pip/pip-452.md

@@ -12,6 +12,9 @@ Inefficient Filtering: For namespaces with millions of topics, the broker must f
 To address these issues, I propose making the topic listing logic pluggable and extending the protocol to accept client properties.
 
 # Goals
+
+## In Scope
+
 Protocol: Add a properties field to `CommandGetTopicsOfNamespace` to carry client-side context.
 
 Broker: Extend `BrokerInterceptor` to allow plugins to intercept the "Get Topics" request.
@@ -20,6 +23,13 @@ Client: Update the Java Client to forward Consumer properties to the lookup serv
 
 Admin API & CLI: Update the REST API and CLI to accept properties for listing topics in a namespace.
 
+Configuration: Add a broker configuration to enable/disable topic list watching.
+
+## Out of Scope
+
+Not support topic list watching for customized topic listing in this PIP. It can be considered in future work.
+If you want to use this feature, you need to disable topic list watching by set `enableBrokerTopicListWatcher = false`.
+
 # High Level Design
 We will modify the Pulsar Protocol to carry a properties map. On the Broker side, we will add a new method to the BrokerInterceptor interface.
 The default implementation will preserve the existing behavior (fetching from ZooKeeper). The Broker's connection handler will simply delegate the request to this strategy.
@@ -28,6 +38,14 @@ The default implementation will preserve the existing behavior (fetching from Zo
 
 ## Public-facing Changes
 
+### Configuration
+
+broker.conf
+```properties
+# Enables watching topic add/remove events on broker side. It is separated from enableBrokerSideSubscriptionPatternEvaluation.
+enableBrokerTopicListWatcher = true
+```
+
 ### Protocol Changes
 Update `PulsarApi.proto` to include the properties field.