Fixes fluentd new push path API.

[cyriltovena]

What this PR does / why we need it:

Fixes the current 1.2.3 fluentd Loki plugin. We change the push path (#1326) but the API has actually change.

I have not bump the version as I'm planning to replace the current not working released version.

I've also:

• fixed all style/lint issue. (I'd like to keep it that way !! I'll be more careful with review)
• Added more tests to verify ordering
• Improved nanoseconds precision by using nsec instead of to_f (see . https://ruby-doc.org/core-2.6.4/Time.html#method-i-nsec)

Output of rake:

rake spec
Running RuboCop...
Inspecting 7 files
.......

7 files inspected, no offenses detected
/System/Library/Frameworks/Ruby.framework/Versions/2.6/usr/bin/ruby -I/Library/Ruby/Gems/2.6.0/gems/rspec-support-3.8.2/lib:/Library/Ruby/Gems/2.6.0/gems/rspec-core-3.8.2/lib /Library/Ruby/Gems/2.6.0/gems/rspec-core-3.8.2/exe/rspec --pattern spec/\*\*\{,/\*/\*\*\}/\*_spec.rb
SimpleCov started successfully!

Fluent::Plugin::LokiOutput
  loads config
  converts syslog output to loki output
  converts syslog output with extra labels to loki output
  converts multiple syslog output lines to loki output
  converts multiple syslog output lines with extra labels to loki output
  formats record hash as key_value
  formats record hash as json
  extracts record key as label
  extracts nested record key as label
  extracts nested record key as label and drop key after
  formats as simple string when only 1 record key
  order by timestamp then index when received unordered

Finished in 0.02656 seconds (files took 1.04 seconds to load)
12 examples, 0 failures

Coverage report generated for RSpec to /Users/ctovena/go/src/github.com/grafana/loki/fluentd/fluent-plugin-grafana-loki/coverage. 148 / 148 LOC (100.0%) covered.

Which issue(s) this PR fixes:
Fixes #1347

Special notes for your reviewer:

Checklist

• [x ] Tests updated

[cyriltovena]

/cc @tarokkk @dawidmalina

[dawidmalina]

LGTM

[tarokkk]

Looks much better 👍 Thanks for the lint work as well ;)

[tarokkk]

Although I think we need to update gem version

[cyriltovena]

Although I think we need to update gem version

No idea if I can force push, but I guess it's cleaner to bump it.

[tarokkk]

I mean we need to bump it, you can't force push gems on the gem repo

[cyriltovena]

well I didn't know, will do ! Thanks