Discard the millisecond of the Time value

When it fails to serialize data to MsgPack format,
it falls back to part using `JSON.parse(JSON.generate(msg)).to_msgpack`.

https://github.com/fluent/fluent-logger-ruby/blob/6bcebac554ceb8d5368fbfa128336fa4b49c8225/lib/fluent/logger/fluent_logger.rb#L233-L244

The fallback part discard the millisecond of the Time value.
There are cases where people dislike it.

Example:

```ruby
require "bundler/inline"
gemfile do
source "https://rubygems.org"
gem "fluent-logger"
gem "activesupport"
end

require 'active_support/json'

log = Fluent::Logger::FluentLogger.new(nil, :host => 'localhost', :port => 24224)

log.post("myapp.access", { user: "foo", created_at: Time.now })
log.post("myapp.access", { user: "foo", created_at: Time.now.as_json })
```

Beforehand, it preserves milliseconds converting with `as_json`.
(If active_support is loaded, `to_json` will invoke `as_json` automatically.)

This behavior is preferred by default.

```
2025-09-19 11:54:11.000000000 +0900 incoming: {"user":"foo","created_at":"2025-09-19 11:54:11 +0900"}
2025-09-19 11:54:11.000000000 +0900 incoming: {"user":"foo","created_at":"2025-09-19T11:54:11.828+09:00"}
```

We need to regist custom MsgPack packer for Time object, or something...