Limit high-memory logging ('pattern not matched' in parsing) to necessary log level

[Watson1978]

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.

[kenhys]

LGTM.

[daipom]

Thanks!

Note: Long strings are allocated on the heap, so they are likely not released immediately.
It would be better not to generate unnecessary long strings.
(It probably won't affect normal use cases, though)