in_tail: Show more information on skipping update_watcher

In #3614 some users reported that in_tail rarely stops tailing. It seems
it's caused when skipping update_watcher due to unexpected duplicate
pos_file entries. The root cause is unknown yet. To investigate it, show
more infomation when it happens. Ideally it should't happen so that log
level "warn" is desired for it.

Signed-off-by: Takuro Ashie <[email protected]>

lib/fluent/plugin/in_tail.rb

@@ -498,7 +498,8 @@ def update_watcher(target_info, pe)
         pe_inode = pe.read_inode
         target_info_from_position_entry = TargetInfo.new(path, pe_inode)
         unless pe_inode == @pf[target_info_from_position_entry].read_inode
-          log.debug "Skip update_watcher because watcher has been already updated by other inotify event"
+          log.warn "Skip update_watcher because watcher has been already updated by other inotify event",
+                   path: path, inode: pe.read_inode, inode_in_pos_file: @pf[target_info_from_position_entry].read_inode
           return
         end
       end