in_debug_agent: accept only from local machine by default (#5272)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 

Usually, in_debug_agent must be explicitly enabled by users who know
what you do.
But, there is an security concern which accepts external access by
default even though user must enable it explicitly.

With this commit, change that behavior a bit secure by default.

**Docs Changes**:

Need to update default.


https://docs.fluentd.org/monitoring-fluentd/monitoring-rest-api#datadog-dd-agent-integration

**Release Note**: 

N/A

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

lib/fluent/plugin/in_debug_agent.rb

@@ -26,7 +26,7 @@ def initialize
       super
     end
 
-    config_param :bind, :string, default: '0.0.0.0'
+    config_param :bind, :string, default: '127.0.0.1'
     config_param :port, :integer, default: 24230
     config_param :unix_path, :string, default: nil
     #config_param :unix_mode  # TODO

test/plugin/test_in_debug_agent.rb

@@ -46,4 +46,12 @@ def test_multi_worker_environment_with_unix_path
       assert_false d.instance.multi_workers_ready?
     end
   end
+
+  def test_default_configuration
+    assert_nothing_raised do
+      d = create_driver
+      assert_equal(['127.0.0.1', 24230, 'Fluent::Engine'],
+                   [d.instance.bind, d.instance.port, d.instance.object])
+    end
+  end
 end