in_http: Add support for HTTP GET requests

lib/fluent/plugin/in_http.rb

@@ -461,6 +461,12 @@ def on_body(chunk)
       RES_200_STATUS = "200 OK".freeze
       RES_403_STATUS = "403 Forbidden".freeze
 
+      # Azure App Service sends GET requests for health checking purpose.
+      # Respond with `200 OK` to accommodate it.
+      def handle_get_request
+          return send_response_and_close(RES_200_STATUS, {}, "")
+      end
+
       # Web browsers can send an OPTIONS request before performing POST
       # to check if cross-origin requests are supported.
       def handle_options_request
@@ -494,6 +500,10 @@ def handle_options_request
       def on_message_complete
         return if closing?
 
+        if @parser.http_method == 'GET'.freeze
+          return handle_get_request()
+        end
+
         if @parser.http_method == 'OPTIONS'.freeze
           return handle_options_request()
         end

test/plugin/test_in_http.rb

@@ -770,6 +770,15 @@ def test_cors_allowed_wildcard
     end
   end
 
+  def test_get_request
+    d = create_driver(CONFIG)
+
+    d.run do
+      res = get("/cors.test", {}, {})
+      assert_equal "200", res.code
+    end
+  end
+
   def test_cors_preflight
     d = create_driver(CONFIG + 'cors_allow_origins ["*"]')
 
@@ -985,6 +994,12 @@ def test_if_content_type_is_initialized_properly
     assert_equal $test_in_http_connection_object_ids[0], $test_in_http_connection_object_ids[1]
   end
 
+  def get(path, params, header = {})
+    http = Net::HTTP.new("127.0.0.1", PORT)
+    req = Net::HTTP::Get.new(path, header)
+    http.request(req)
+  end
+
   def options(path, params, header = {})
     http = Net::HTTP.new("127.0.0.1", PORT)
     req = Net::HTTP::Options.new(path, header)