Inject :connection to sql.active_record event instead of using #_id2ref

[delner]

As an alternative implementation to #647, this adds :connection to Rails sql.active_record event, then uses that object to derive connection configuration for the trace. To accomplish this, it leverages #648 to add a Datadog::Shim::Double into the AbstractAdapter class.

:connection has recently been added to Rails master via rails/rails#34602. The behavior in this PR is for the benefit of Rails versions preceding that change on Rails.

TODO

• Add :connection to ActiveRecord event
• Change ActiveRecord integration to use :connection
• Add specs
• Compare performance

[delner]

I created a pretty simple performance_spec.txt which runs connection.execute('SELECT 42') 100,000 times against an in-memory database and used it to to measure both implementations. Each had comparable performance, with Shim::Double just being a slight bit faster, but both are a bit slower than the control:

Control commit b67871fbde052d36a44e1e7fa2747bce7c2f3c29:

root@d6dc35cafbcb:/app# bundle exec appraisal contrib rspec spec/ddtrace/contrib/active_record/performance_spec.rb
>> BUNDLE_GEMFILE=/app/gemfiles/contrib.gemfile bundle exec rspec spec/ddtrace/contrib/active_record/performance_spec.rb

Randomized with seed 53122

Run time for 100000 iterations: 8.59
.

Finished in 10.45 seconds (files took 0.83915 seconds to load)
1 example, 0 failures

Randomized with seed 53122

Use of Shim::Double and injecting connection arg in this PR commit 9e12665356be00603f153ebe7b8b988a694534a9:

root@d43f6b42218a:/app# bundle exec appraisal contrib rspec spec/ddtrace/contrib/active_record/performance_spec.rb 
>> BUNDLE_GEMFILE=/app/gemfiles/contrib.gemfile bundle exec rspec spec/ddtrace/contrib/active_record/performance_spec.rb

Randomized with seed 39954

Run time for 100000 iterations: 8.93
.

Finished in 10.84 seconds (files took 0.84327 seconds to load)
1 example, 0 failures

Randomized with seed 39954

Using iterative strategy, by searching connections in the connection pool in #647 commit e2613ee11565ebed99e60dc5899f735a75339e57:

root@cb5b6a204da8:/app# bundle exec appraisal contrib rspec spec/ddtrace/contrib/active_record/performance_spec.rb
>> BUNDLE_GEMFILE=/app/gemfiles/contrib.gemfile bundle exec rspec spec/ddtrace/contrib/active_record/performance_spec.rb

Randomized with seed 47628

Run time for 100000 iterations: 8.97
.

Finished in 10.72 seconds (files took 0.86328 seconds to load)
1 example, 0 failures

Randomized with seed 47628

We should probably look at more qualitative differences to determine what's the best strategy, and see if there's something we can do to bring performance closer to the control.

[pawelchcki]

Lookin at the benchmarks I'm s leaning a bit more towards the solution that uses the object proxy/shim. As it seems to me to be a bit more versatile.

[delner]

Cool. Need to rebase this on the shim PR then after I add specs there, then I'll wrap this one up.