Tracing subscribers should be multi-event based

[st0012]

This will make future extension (like #1558) easier to implement.

[codecov-commenter]

Codecov Report

Merging #1587 (5eb4686) into master (53deb6e) will decrease coverage by 0.02%.
The diff coverage is 71.42%.

❗ Current head 5eb4686 differs from pull request most recent head b0d2aa9. Consider uploading reports for the commit b0d2aa9 to get more accurate results

@@            Coverage Diff             @@
##           master    #1587      +/-   ##
==========================================
- Coverage   98.47%   98.44%   -0.03%     
==========================================
  Files         127      127              
  Lines        7091     7095       +4     
==========================================
+ Hits         6983     6985       +2     
- Misses        108      110       +2     

Impacted Files	Coverage Δ
...ls/lib/sentry/rails/tracing/abstract_subscriber.rb	75.86% <53.84%> (-4.91%)	⬇️
sentry-rails/lib/sentry/rails/tracing.rb	97.36% <100.00%> (+0.07%)	⬆️
...ntry/rails/tracing/action_controller_subscriber.rb	89.47% <100.00%> (ø)
...lib/sentry/rails/tracing/action_view_subscriber.rb	100.00% <100.00%> (ø)
...b/sentry/rails/tracing/active_record_subscriber.rb	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 53deb6e...b0d2aa9. Read the comment docs.