include http status code in metadata

[yaauie]

Release notes

The HTTP status code for each request is now included in the response metadata.

What does this PR do?

Adds HTTP status code of response

Why is it important/What is the impact to the user?

Adds HTTP status code of response, so that a pipeline can differentiate between the semantics of different error status codes. For example, in some contexts a 404 (Not Found) is substantially different than a 400 (Bad Request), and it may be helpful for downstream plugins in a pipeline to handle the two cases separately.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Related issues

• Resolves: HTTP Filter Plugin - Request/Response Metadata elastic/logstash#14368