Support dynamic placeholder for out_http headers parameter

[davidbhoward]

Check CONTRIBUTING guideline first and here is the list to help us investigate the problem.

Is your feature request related to a problem? Please describe.

Today I can't use dynamic headers for out_http plugin.

Describe the solution you'd like

We need the ability for headers to be dynamic in nature-based upon values inside the record. Today they can only be strings, but the goal would be to do something like header X-Custom-Host:${$.kubernetes.host},X-Custom-Datacenter:${$.datacenter}

Describe alternatives you've considered

I have tried all configuration options I could think with no luck. After exhausting all options, I finally reached out to the community and got a response from @repeatedly saying:

Currently, out_http's headers parameter doesn't support dynamic placeholder.
For it, we need to add headers_from_record like parameter with buffer placeholder.

Additional context

The question was posted on the Fluentd Google Group with the subject, "out_http headers not dynamic".

[davidbhoward]

Thank you, @repeatedly, for doing this so quickly. We are already using it and it is working very well. Thanks again.