http_in plugin respond with 200 OK to GET /

[JoshKeegan]

Is your feature request related to a problem? Please describe.
I am trying to run Fluentd by deploying a docker container to an Azure App Service. App Service is designed for running HTTP APIs and websites and expects an application to respond with HTTP 200 to GET / before it will route any requests to it.

Describe the solution you'd like
I would like to have http_in respond with 200 OK to a GET / request.

Describe alternatives you've considered
Running the http_in plugin behind nginx within the same container andn having nginx handle GET /. This isn't preferred for a few reasons though.

Additional context
N/A

Is this reasonable? I am happy to work on it & open a PR.

[fujimotos]

I would like to have http_in respond with 200 OK to a GET / request.

@JoshKeegan Should be fixed by #3373.

[JoshKeegan]

I would like to have http_in respond with 200 OK to a GET / request.

@JoshKeegan Should be fixed by #3373.

Thankyou for the quick response & PR!