Fix: Get connection from connection_pool instead of ActiveRecord::Base.connection

[Iwaide]

Issue

When attempting to retrieve a connection in ActiveRecordSubscriber, a loop occurs, eventually leading to ActiveRecord::ConnectionTimeoutError.

In versions of Rails lower than 6, since the payload does not contain a connection, it tries to use ActiveRecord::Base.connection to get the current connection. However, it seems that this causes subscribe! to be called again.
I am not sure if this issue is specific to MySQL. Please refer to the following repository for a reproduction environment:
https://github.com/Iwaide/re_app_for_sentry/

Fix

I have modified the code to search for and retrieve a connection from ActiveRecord::Base.connection_pool.connections.
I am not very familiar with connection management, so I am not entirely confident that this is the appropriate approach. Please make any necessary corrections.

[codecov]

Codecov Report

Merging #2278 (11a1f51) into master (7d29d3c) will increase coverage by 0.02%.
Report is 2 commits behind head on master.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2278      +/-   ##
==========================================
+ Coverage   97.61%   97.64%   +0.02%     
==========================================
  Files         112      112              
  Lines        4155     4154       -1     
==========================================
  Hits         4056     4056              
+ Misses         99       98       -1     

Components	Coverage Δ
sentry-ruby	98.30% <ø> (-0.04%)	⬇️
sentry-rails	95.22% <0.00%> (+0.17%)	⬆️
sentry-sidekiq	94.70% <ø> (ø)
sentry-resque	92.30% <ø> (+1.53%)	⬆️
sentry-delayed_job	95.60% <ø> (ø)
sentry-opentelemetry	100.00% <ø> (ø)

Files	Coverage Δ
...b/sentry/rails/tracing/active_record_subscriber.rb	85.71% <0.00%> (+2.95%)	⬆️

... and 2 files with indirect coverage changes

[sl0thentr0py]

thanks @Iwaide !