Increase max event size to 1MB (#265)

AWS has increased the max event size to 1 MB: [AWS News](https://aws.amazon.com/about-aws/whats-new/2025/04/amazon-cloudwatch-logs-increases-log-event-size-1-mb/#:~:text=Amazon%20CloudWatch%20Logs%20now%20supports,visit%20the%20CloudWatch%20Logs%20documentation.)

Author: amykelly

README.md

@@ -328,7 +328,7 @@ rake test
 
 ## Caution
 
-If an event message exceeds API limit (256KB), the event will be discarded.
+If an event message exceeds API limit (1MB), the event will be discarded.
 
 ## Cross-Account Operation
 

lib/fluent/plugin/out_cloudwatch_logs.rb

@@ -68,7 +68,7 @@ class TooLargeEventError < Fluent::UnrecoverableError; end
     end
 
     MAX_EVENTS_SIZE = 1_048_576
-    MAX_EVENT_SIZE = 256 * 1024
+    MAX_EVENT_SIZE = 1024 * 1024
     EVENT_HEADER_SIZE = 26
 
     def initialize

test/plugin/test_out_cloudwatch_logs.rb

@@ -974,11 +974,11 @@ def test_too_large_event
         @log_level debug
       EOC
       d.run(default_tag: fluentd_tag) do
-        d.feed(time, {'message' => '*' * 256 * 1024})
+        d.feed(time, {'message' => '*' * 1024 * 1024})
       end
 
       logs = d.logs
-      assert(logs.any?{|log| log =~ /Log event in .* discarded because it is too large: 262184 bytes exceeds limit of 262144/})
+      assert(logs.any?{|log| log =~ /Log event in .* discarded because it is too large/})
     end
 
     def test_do_not_emit_empty_record