Show request headers instead of object

[kaylareopelle]

When a customer uses the HTTPClient library for their external requests, they do not see the headers for the requests in the logs, but instead the human-readable representation of the object.

Example:
OUTGOING REQUEST HEADERS: #<NewRelic::Agent::HTTPClients::HTTPClientRequest:0x00007fd0dda983e0>

The content following the colon should be a list of headers.

The DistributedTracer#log_request_headers method is responsible for logging this line of code. It expects the first argument, headers will always be an interpolatable value that includes the header values.

This method is invoked by ExternalRequestSegment#add_request_headers, which calls DistributedTracer#insert_headers with the request object as an argument, followed by DistributedTracer#log_request_headers.

DistributedTracer#insert_headers uses the argument it receives from ExternalRequestSegment#add_request_headers for multiple method calls:

• insert_trace_context_headers
• insert_distributed_trace_header
• insert_cross_app_header
• log_request_headers

It's unclear if these method calls are silently failing when the external request is an HTTPClientRequest.

Acceptance Criteria

• Logged headers for HTTPClientRequest objects are the headers and not the human-readable representation of the HTTPClientRequest object
• All HTTP client libraries have their headers correctly logged
• Methods called in DistributedTracer#insert_headers are able to do what they need to with the header argument for each external request library (make sure there aren't silent failures happening when other DT headers are getting added)

[workato-integration]

https://issues.newrelic.com/browse/NEWRELIC-5612

[kaylareopelle]

Sidekick please, @tannalynn!

[fallwith]

Hey team! Please add your planning poker estimate with Zenhub @hannahramadan @kaylareopelle @tannalynn

[angelatan2]

From grooming: we may need to add a test to cover headers on the HTTP clients.

[fallwith]

Released in v9.1.0 of the newrelic_rpm gem.