output: add metrics in delayed_commit

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/plugin/output.rb

@@ -1214,6 +1214,7 @@ def try_flush
           if output.delayed_commit
             log.trace "executing delayed write and commit", chunk: dump_unique_id_hex(chunk.unique_id)
             @write_count_metrics.inc
+            @write_secondary_count_metrics.inc if using_secondary
             @dequeued_chunks_mutex.synchronize do
               # delayed_commit_timeout for secondary is configured in <buffer> of primary (<secondary> don't get <buffer>)
               @dequeued_chunks << DequeuedChunkInfo.new(chunk.unique_id, Time.now, self.delayed_commit_timeout)

test/plugin/test_output_as_buffered_secondary.rb

@@ -446,6 +446,8 @@ def dummy_event_stream
       assert{ @i.buffer.dequeued[chunks[0].unique_id].nil? }
       assert{ chunks.first.empty? }
 
+      assert{ @i.instance_variable_get(:@write_secondary_count_metrics).get > 0 }
+
       assert_nil @i.retry
 
       logs = @i.log.out.logs