fix: add headers to honeycomb event

[JamieDanielson]

Which problem is this PR solving?

• Closes Events in Honeycomb don't include specified headers #288

Short description of the changes

• add fields for http.request.headers and http.response.headers and accompanying unit tests. feat: Add configurable list of HTTP headers to extract #277 added all the logic for configuring the HTTP_HEADERS to be extracted, and this PR adds the headers to the actual event to be sent to Honeycomb.

This is the smaller implementation mentioned in the issue. We could (/should?) have individual attributes for each key, but I figured this could be a faster fix to start since we've already published with the expectation of HTTP_HEADERS working.

How to verify that this has the expected result

Run the agent with something like the example greeting service and see the attributes in Honeycomb.

The default captures just the User-Agent:

• http.request.headers
  • example: "{\"User-Agent\":[\"Go-http-client/1.1\"]}"
• http.response.headers
  • example: "{}"

If for example the list includes "User-Agent,Accept-Encoding,Content-Length,Content-Type", the result looks like this:

• http.request.headers
  • example: "{\"Accept-Encoding\":[\"gzip\"],\"User-Agent\":[\"Go-http-client/1.1\"]}"
• http.response.headers
  • example: "{\"Content-Length\":[\"12\"],\"Content-Type\":[\"text/plain; charset=utf-8\"]}"