Convert second(Integer class) to nanosecond precision

[takanabe]

What this PR does / why we need it:

The commits to convert second precision to nanosecond prevision should be considered not only Fluent::EventTime object but Integer object. Otherwise, this causes the following error.

td-agent.log:482992:2020-01-30 03:40:48 +0000 [warn]: #0 [forward_to_loki] got unrecoverable error in primary and no secondary error_class=NoMethodError error="undefined method `nsec' for 1580355636:Integer\nDid you mean?  inspect"
td-agent.log:482994:2020-01-30 03:40:48 +0000 [warn]: #0 [forward_to_loki] bad chunk is moved to /tmp/fluent/backup/worker0/forward_to_loki/59d53385f435fa0b70e60a0101e7091d.log

This is because Integer does not have nsec method

irb(main):001:0> 1.class
=> Integer
irb(main):002:0> 1.nsec
Traceback (most recent call last):
        4: from /Users/takayuki-watanabe/.rbenv/versions/2.6/bin/irb:23:in `<main>'
        3: from /Users/takayuki-watanabe/.rbenv/versions/2.6/bin/irb:23:in `load'
        2: from /Users/takayuki-watanabe/.rbenv/versions/2.6.5/lib/ruby/gems/2.6.0/gems/irb-1.2.0/exe/irb:11:in `<top (required)>'
        1: from (irb):2
NoMethodError (undefined method `nsec' for 1:Integer)

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

This PR should be merged after I add some specs but it will take time since the current specs are broken. #1645

Checklist

• Tests updated: Not yet

[cyriltovena]

LGTM

[nomatterz]

Events from in_forward plugin are not getting to the Loki without this fix.
Nice job!
Is there a way to change time type in original event using default fluentd plugins ( so it's not Integer)? Or there should be strict alignment between input and output plugins?

[cyriltovena]

I still need to push the gem, for this PR.