Tag an ActiveRecord span as cached

[kkumler]

When digging through a transaction, we could not readily confirm if a repeated ActiveRecord query was using the cache or not. The information is already sent along with the sql.active_record instrumentation since Rails 5.1 but only from the QueryCache origin.
The tag is added only if cached: true is in the payload.

[codecov-commenter]

Codecov Report

Base: 80.16% // Head: 98.01% // Increases project coverage by +17.84% 🎉

Coverage data is based on head (ef1baae) compared to base (81b0f9b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #1968       +/-   ##
===========================================
+ Coverage   80.16%   98.01%   +17.84%     
===========================================
  Files         116      157       +41     
  Lines        5284     9862     +4578     
===========================================
+ Hits         4236     9666     +5430     
+ Misses       1048      196      -852     

Impacted Files	Coverage Δ
...b/sentry/rails/tracing/active_record_subscriber.rb	100.00% <100.00%> (ø)
...try/rails/tracing/active_record_subscriber_spec.rb	100.00% <100.00%> (ø)
sentry-resque/lib/sentry/resque.rb	97.22% <0.00%> (-2.78%)	⬇️
sentry-rails/lib/sentry/rails/action_cable.rb	100.00% <0.00%> (ø)
...entry-rails/spec/sentry/rails/action_cable_spec.rb	100.00% <0.00%> (ø)
sentry-ruby/spec/sentry/transaction_spec.rb	100.00% <0.00%> (ø)
sentry-ruby/spec/sentry/scope_spec.rb	100.00% <0.00%> (ø)
sentry-ruby/spec/sentry/utils/real_ip_spec.rb	100.00% <0.00%> (ø)
sentry-ruby/spec/sentry/rake_spec.rb	100.00% <0.00%> (ø)
sentry-ruby/spec/sentry/span_spec.rb	100.00% <0.00%> (ø)
... and 79 more

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[st0012]

Thanks for this nice addition 👍