[NO-TICKET] Fix error during telemetry debug logging attempt

[ivoanjo]

What does this PR do?

This PR fixes the following debug error log that would happen when enabling debug logging + not having an agent running locally.

$ DD_TRACE_DEBUG=true bundle exec ruby -e "require 'datadog'; Datadog.configure {}; sleep 1"
...
DEBUG -- datadog: [datadog] (lib/datadog/core/telemetry/http/adapters/net.rb:47:in `rescue in post') Unable to send telemetry event to agent: Failed to open TCP connection to 127.0.0.1:8126 (Connection refused - connect(2) for "127.0.0.1" port 8126)
DEBUG -- datadog: [datadog] (lib/datadog/core/telemetry/emitter.rb:32:in `rescue in request') Unable to send telemetry request for event `app-started`: undefined method `code' for Datadog::Core::Telemetry::Http::InternalErrorResponse ok?: unsupported?:, not_found?:, client_error?:, server_error?:, internal_error?:true, payload:, error_type:Errno::ECONNREFUSED error:Failed to open TCP connection to 127.0.0.1:8126 (Connection refused - connect(2) for "127.0.0.1" port 8126):Datadog::Core::Telemetry::Http::InternalErrorResponse

            Datadog.logger.debug { "Telemetry sent for event `#{event.type}` (status code: #{res.code})" }
                                                                                                ^^^^^

Specifically, the class used to communicate errors (Datadog::Core::Telemetry::Http::Response::InternalErrorResponse) did not define a #code accessor, thus making the log message that tries to debug the response code fail.

Motivation:

Fix this error.

Additional Notes:

The error itself was annoying, but harmless, as we did catch the exception anyway.

I think the current approach of having a Response module that then gets inserted into a few classes is a bit error prone.

And this is one where we definitely would've been saved by having type signatures for this part of the code.

How to test the change?

I've added unit coverage + you can use the example above to confirm it no longer happens manually.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.13%. Comparing base (2be8d89) to head (a73b051).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3617   +/-   ##
=======================================
  Coverage   98.12%   98.13%           
=======================================
  Files        1223     1223           
  Lines       72136    72146   +10     
  Branches     3425     3425           
=======================================
+ Hits        70787    70797   +10     
  Misses       1349     1349           

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