formatter_json: reduce memory usage (#4886)

**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 .


![chart](https://github.com/user-attachments/assets/2a179770-a99f-45c0-bf60-797c567e9505)

* 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**:

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

lib/fluent/plugin/formatter_json.rb

@@ -48,7 +48,10 @@ def configure(conf)
       end
 
       def format(tag, time, record)
-        "#{@dump_proc.call(record)}#{@newline}"
+        json_str = @dump_proc.call(record)
+        "#{json_str}#{@newline}"
+      ensure
+        json_str&.clear
       end
 
       def format_without_nl(tag, time, record)