time_key default needlessly overridden in json, regexp, and ltsv parsers

Describe the bug The global default for time_key is nil, indicating that the event time should not be derived from the message at all, but the json, regexp, and ltsv parsers override this to "time". Furthermore, the fluentd config syntax does not permit time_key to be set back to nil. as specifying nil, null, or "" result in the literal string representations "nil", "null", and "" being used. Specifying "#{use_nil}" results in an error that a value is required.

While it appears that time_key was required to be defined in the original implementations of these parsers, it has not been required for some time now, and the result is an edge case where a message contains a field called time that is not parseable as time causes an exception and the message to be dropped.

It is possible to work around this issue by setting time_key to a value that you do not expect to occur as a key in your messages, but this is a kludge at best.

It's also worth noting here that the docs do note that time_key is overridden by these parsers: https://docs.fluentd.org/configuration/parse-section#parse-parameters

To Reproduce

1. Define any input with a json parser.
2. Submit a message such as {"time": {"begin": 1611709938020}}

Expected behavior The message to be decoded as a dict with a key named "time" and its arbitrary value.

Environment

• Fluentd or td-agent version: 1.11.5
• Operating system: docker image fluent/fluentd:v1.11.5-1.0

Configuration

<source>
  # ...
  <parse>
    @type json
  </parse>
</source>

Error Log

#<Fluent::Plugin::Parser::ParserError: value must be a string or a number: {"client_sent"=>1611709938020}(Hash)> @ /usr/lib/ruby/gems/2.7.0/gems/fluentd-1.11.5/lib/fluent/plugin/parser.rb:196:in `rescue in parse_time'