in_tail: add metrics to know file count monitored by TailWatcher

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

lib/fluent/plugin/in_tail.rb

@@ -814,6 +814,7 @@ def statistics
           'closed_file_count' => @metrics.closed.get,
           'rotated_file_count' => @metrics.rotated.get,
           'throttled_log_count' => @metrics.throttled.get,
+          'tracking_file_count' => @tails.size,
         })
       }
       stats

test/plugin/test_in_tail.rb

@@ -3443,4 +3443,42 @@ def test_next_rotation_occurs_very_fast_while_old_TW_still_waiting_rotate_wait
                     d.logs[-2..]
                   ])
   end
+
+  test 'statistics' do
+    config = config_element("", "", {
+      "tag" => "statistics",
+      "path" => "#{@tmp_dir}/statistics*.txt",
+      "format" => "none",
+      "read_from_head" => true,
+    })
+    Fluent::FileWrapper.open("#{@tmp_dir}/statistics1.txt", "w+") do |f|
+      f.puts "foo"
+    end
+
+    d = create_driver(config, false)
+    d.run(expect_records: 1, shutdown: false)
+
+    assert_equal({
+                   "emit_records" => 0,
+                   "emit_size" => 0,
+                   "opened_file_count" => 1,
+                   "closed_file_count" => 0,
+                   "rotated_file_count" => 0,
+                   "throttled_log_count" =>0,
+                   "tracking_file_count" => 1,
+                 },
+                 d.instance.statistics["input"])
+
+    d.instance_shutdown
+
+    assert_equal({
+                   "emit_records" => 0,
+                   "emit_size" => 0,
+                   "opened_file_count" => 1,
+                   "closed_file_count" => 1,
+                   "rotated_file_count" => 0,
+                   "throttled_log_count" =>0,
+                   "tracking_file_count" => 0,
+                 },
+                 d.instance.statistics["input"])  end
 end