samples(pubsub): Add publisher flow control sample

closes: #11685
pr: #12005

google-cloud-pubsub/samples/acceptance/topics_test.rb

@@ -317,6 +317,17 @@
     end
   end
 
+  it "supports pubsub_publisher_flow_control" do
+    #setup
+    @topic = pubsub.create_topic topic_id
+    @subscription = @topic.subscribe random_subscription_id
+
+    # pubsub_publisher_flow_control
+    assert_output "Published messages with flow control settings to #{topic_id}.\n" do
+      publish_messages_async_with_flow_control topic_id: topic_id
+    end
+  end
+
   it "supports publish_with_error_handler" do
     #setup
     @topic = pubsub.create_topic topic_id

google-cloud-pubsub/samples/topics.rb

@@ -290,6 +290,37 @@ def publish_messages_async_with_concurrency_control topic_id:
   # [END pubsub_publisher_concurrency_control]
 end
 
+def publish_messages_async_with_flow_control topic_id:
+  # [START pubsub_publisher_flow_control]
+  # topic_id = "your-topic-id"
+  require "google/cloud/pubsub"
+
+  pubsub = Google::Cloud::Pubsub.new
+
+  topic = pubsub.topic topic_id, async: {
+    # Configure how many messages the publisher client can hold in memory
+    # and what to do when messages exceed the limit.
+    flow_control: {
+      message_limit: 100,
+      byte_limit: 10 * 1024 * 1024, # 10 MiB
+      # Block more messages from being published when the limit is reached. The
+      # other options are :ignore and :error.
+      limit_exceeded_behavior: :block
+    }
+  }
+  # Rapidly publishing 1000 messages in a loop may be constrained by flow control.
+  1000.times do |i|
+    topic.publish_async "message #{i}" do |result|
+      raise "Failed to publish the message." unless result.succeeded?
+    end
+  end
+
+  # Stop the async_publisher to send all queued messages immediately.
+  topic.async_publisher.stop.wait!
+  puts "Published messages with flow control settings to #{topic_id}."
+  # [END pubsub_publisher_flow_control]
+end
+
 def publish_with_error_handler topic_id:
   # [START pubsub_publish_with_error_handler]
   # topic_id = "your-topic-id"