HTTP spy incompatible with version http.rb 6.x

[janko]

Describe the bug

The spy for http.rb gem isn't compatible with its 6.x releases.

Steps to reproduce

Try to make a request using http.rb gem, and get this error:

NoMethodError: undefined method '[]=' for an instance of HTTP::Request (NoMethodError)
  trace_context.apply_headers { |key, value| req[key] = value }

Version 6.x removed [] and []= as a shorthand for headers on HTTP::Request object.

Expected behavior

No errors.

Environment

• OS:
• Ruby version:
• Framework and version:
• APM Server version:
• Agent version:

Additional context

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