filter_parser: add error event for multiple parsed results

[daipom]

Which issue(s) this PR fixes:

• Following to Make sure parser returns hash #4474
• Related to JSON parser plugin can sometimes output records as strings rather than Hash #4100

What this PR does / why we need it:
The following problem of filter_parser remains in #4474 :

However, we have a remaining problem with filter_parser.
filter_parser could return Array based on this wrong behavior.
This PR disables it, so it can not return multiple parsed results anymore.
Even though it was wrong to begin with, it's possible that this change in specifications would be unacceptable.
We need to consider this change carefully.
I explain some examples in detail below.

At least, it would not be happy if some events were lost without any warnings or errors.
So, this PR adds an error handling for that.

Docs Changes:
TODO

Release Note:
Same as the title.

[daipom]

Sample:

<source>
  @type sample
  tag test.array
  sample {"message": "[{\"k\":\"v\"}, {\"k2\":\"v2\"}]"}
</source>

<filter test.**>
  @type parser
  key_name message
  <parse>
    @type json
  </parse>
</filter>

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

2024-04-30 14:35:58 +0900 [warn]: #0 dump an error event: error_class=Fluent::Plugin::Parser::ParserError error="Could not emit the event. The parser returned multiple results, but currently filter_parser plugin only returns the first parsed result. Raw data: '[{\"k\":\"v\"}, {\"k2\":\"v2\"}]'" location=nil tag="test.array" time=2024-04-30 14:35:58.020965248 +0900 record={"k2"=>"v2"}
2024-04-30 14:35:58.020948041 +0900 test.array: {"k":"v"}

The warning message:

2024-04-30 14:35:58 +0900 [warn]: #0 dump an error event: error_class=Fluent::Plugin::Parser::ParserError
 error="Could not emit the event. The parser returned multiple results,
 but currently filter_parser plugin only returns the first parsed result.
 Raw data: '[{\"k\":\"v\"}, {\"k2\":\"v2\"}]'"
 location=nil tag="test.array" time=2024-04-30 14:35:58.020965248 +0900 record={"k2"=>"v2"}

[ashie]

LGTM