[NO-TICKET] RFC: Add logger gem as dependency to prepare for future Ruby versions

[ivoanjo]

What does this PR do?

This PR declares a dependency from the datadog gem to the logger gem.

While every PR is an RFC, I explicitly marked this one as such since adding a new dependency should never be done lightly.

Motivation:

This is needed to avoid a warning:

lib/datadog/core/configuration/settings.rb:3: warning: logger was
loaded from the standard library, but will no longer be part of the
default gems starting from Ruby 3.5.0.
You can add logger to your Gemfile or gemspec to silence this warning.

Change log entry

Yes. Add logger gem as dependency to prepare for future Ruby versions

Additional Notes:

logger is just the latest in a long line of gems that have been refactored out in this way. See
ruby/ruby@d7e558e and https://stdgems.org/ (not yet updated for 3.5 as of this writing).

While in the past we've avoided adding extra dependencies due to this extraction work, I think this one does make sense to add:

1. It's supported on every Ruby version we currently support (>= 2.5)
2. It's not a small amount of code to shed/rewrite. And on the other side, I don't see a strong reason for embedding instead of depending on it.

We're getting warnings for two more gems: ostruct and benchmark.

From our internal discussions, we can probably get rid of our usage of them, rather than promote them to a dependency, so that's why I did not touch those warnings in this PR.

How to test the change?

The following script can be used (on Ruby 3.4 and above) to trigger the warning, and to confirm it's gone once logger is added as a dependency:

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'

  gem 'datadog', path: '.'
end

puts RUBY_DESCRIPTION
require 'datadog'
puts Datadog::VERSION::STRING

Make sure to run with just ruby example.rb and not with bundle exec.

[datadog-datadog-prod-us1]

Datadog Report

Branch report: ivoanjo/add-dependency-on-logger-gem
Commit report: 19af8f5
Test service: dd-trace-rb

✅ 0 Failed, 22129 Passed, 1476 Skipped, 5m 53.46s Total Time

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.74%. Comparing base (ee94c0e) to head (19af8f5).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4257      +/-   ##
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- Coverage   97.74%   97.74%   -0.01%     
==========================================
  Files        1353     1353              
  Lines       82392    82392              
  Branches     4224     4224              
==========================================
- Hits        80533    80532       -1     
- Misses       1859     1860       +1     

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[ivoanjo]

While I don't expect this to have any fallback, changing dependencies always makes me a bit nervous so I'll mark this as do-not-merge until we put the 2.9.0 release out.

This way we can merge it in early during the 2.10 cycle and have a bit longer baking time.