formatter_json: reduce memory usage

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:
When dealing with long logs at fast, it might improve memory efficient to clear the original strings used in string interpolation.

This is similar with #4884 .

• config

<source>
  @type sample
  tag test
  sample {"message": "#{'a' * 100 * 1024}"}
  size 100
</source>

<match **>
  @type file
  path "#{File.expand_path '~/tmp/log'}"
  format json
</match>

Docs Changes:

Release Note:

[daipom]

Thanks!