Support Logging of HTTP Headers

[therealdwright]

As part of enhancing the debugging capabilities for header-based canary demos, we'd like to introduce the ability to log HTTP headers in our server logs.

Background:
When conducting header-based canary demos, validating that the correct headers are being sent and received is crucial. This will enable better troubleshooting and confirmation that the logic based on these headers is functioning as expected.

Proposed Solution:

Introduce an environment variable DISPLAY_HEADERS to toggle header logging.

If DISPLAY_HEADERS is enabled, append the incoming request headers to the current log line for each request. Ensure headers are formatted clearly for easy parsing in the logs.