Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
Currently, the “pattern not matched” warning is always generated regardless of the log_level setting. This warning might use a lot of memory when the log is very long.

To reduce memory usage, this PR will generate warnings only when necessary, according to log_level.

Related to #4994

• config

<system>
  log_level error
</system>

<source>
  @type tail
  tag test
  path "#{File.expand_path '~/tmp/fluentd/access-*.log'}"
  refresh_interval 5s
  read_from_head true
  <parse>
    @type regexp
    expression /^(?<log>a+)+$/
    timeout 0.1s
  </parse>
</source>

<match **>
  @type stdout
</match>

Here is memory usage.

Docs Changes:
Not needed.

Release Note:
Same as the title.