[WIP] Add ActiveRecord SQL caller stack

[sandboxws]

Description

Adds an additional query_caller_stack tag to ActiveRecord spans. The new tag includes the Rails' caller stack that triggered the SQL execution. Including the query caller stack as part of the span significantly improves the ability to trace down slow queries reported by DataDog to the part of the code from which it originated and ultimately fixing the query.

TODO

• [ ] Add specs
• [ ] Add screenshots

[codecov-commenter]

Codecov Report

Merging #1483 (86cba91) into master (c82e5b3) will decrease coverage by 0.00%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1483      +/-   ##
==========================================
- Coverage   98.22%   98.22%   -0.01%     
==========================================
  Files         860      860              
  Lines       41271    41278       +7     
==========================================
+ Hits        40538    40544       +6     
- Misses        733      734       +1     

Impacted Files	Coverage Δ
lib/ddtrace/contrib/active_record/events/sql.rb	97.56% <100.00%> (+0.12%)	:arrow_up:
lib/ddtrace/contrib/active_record/ext.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/active_record/utils.rb	90.90% <100.00%> (+0.90%)	:arrow_up:
spec/ddtrace/contrib/sinatra/activerecord_spec.rb	78.12% <100.00%> (+0.34%)	:arrow_up:
...b/action_pack/action_controller/instrumentation.rb	94.28% <0.00%> (-1.43%)	:arrow_down:
spec/ddtrace/configuration/components_spec.rb	99.80% <0.00%> (-0.01%)	:arrow_down:

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[delner]

Thanks for the pull request @sandboxws. Can you show us a screenshot of this in action? What does the tag look like?

[sandboxws]

@delner of course! Will a screenshot of the Ruby hash output suffice or do you prefer a screenshot from an actual DataDog trace from a live product ( i.e. from a DataDog dashboard )?