Remove CI gem dependency

[TonyCTHsu]

2.0 Upgrade Guide notes

As of 2.0, ddtrace, our CI visibility component has been extracted as a separate gem named datadog-ci, and will no longer be installed together with ddtrace.

If you are using our CI visibility product, you will need to include datadog-ci in the project's Gemfile (learn more about the setup)

gem 'ddtrace', '>= 2'
gem 'datadog-ci'

If you do not want to install datadog-ci, please ensure any CI related configuration under config.ci.* is removed.

  Datadog.configure do |config|
    config.ci.enabled = true
    
    config.ci.instrument :rspec
    config.ci.instrument :cucumber
  end

What does this PR do?

This PR removes the datadog-ci dependency from ddtrace

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4faaf65) 98.07% compared to head (d329ce3) 98.07%.

Additional details and impacted files

@@           Coverage Diff           @@
##              2.0    #3288   +/-   ##
=======================================
  Coverage   98.07%   98.07%           
=======================================
  Files        1240     1240           
  Lines       71978    71997   +19     
  Branches     3391     3392    +1     
=======================================
+ Hits        70589    70610   +21     
+ Misses       1389     1387    -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[ekump]

Just a bit of a nitpick: This PR is against the branch drop-ruby-2.1-2.2-2.3-support. Is that accurate? And if so, is removing CI necessary for dropping 2.1-2.3 support?

[anmarchenko]

Thank you! only a couple of points:

• let's remove RSpec, Minitest, Cucumber integrations from https://github.com/DataDog/dd-trace-rb/blob/master/docs/GettingStarted.md and write there "If you are looking for test frameworks integrations, see datadog-ci gem"
• The recommended way to enable CI visibility is (rspec example):

require 'rspec'
require 'datadog/ci'

# Only activates test instrumentation on CI
if ENV["DD_ENV"] == "ci"
  Datadog.configure do |c|
    # Configures the tracer to ensure results delivery
    c.ci.enabled = true

    # The name of the service or library under test
    c.service = "my-ruby-app"

    # Enables the RSpec instrumentation
    c.ci.instrument :rspec
  end
end

[alai97]

Looks good for docs!

[marcotc]

Is this still relevant @TonyCTHsu @anmarchenko? I think this might have to be moved to the datadog-ci gem instead.

[anmarchenko]

I am not sure what is the meaning of this, maybe it is important for tracer to report the value of ci.enabled to telemetry. Though it will always be false because telemetry is not enabled when CI mode is on, so you might as well remove it

[marcotc]

Looks great!
I only left one comment, but overall it's in great shape! #3288 (comment)

[anmarchenko]

I have only one comment: this section https://github.com/DataDog/dd-trace-rb/blob/master/docs/GettingStarted.md#ci-visibility still exists in dd-trace-rb docs. I would like us to put there text "For CI visibility use the datadog-ci gem".

[anmarchenko]

looks good now, thank you!