[CIAPP-1208] Rework RSpec instrumentation as separate traces for each test

[AdrianLC]

This fixes an issue on ci-app where we do not expect several spans with type: test on the same trace.
We discussed two options and went for this:

• Adding a new span type for rspec example_group, that would be excluded from our indexation and queries.
• Reworking the instrumentation to match how it works on other junit style frameworks, as well as on the JS instrumentation for Jest (which also has a BDD style interface).

The instrumentation now only patches RSpec::Example and we remove any wrapping on ExampleGroup.
This is a new integration only used internally so it should be fine to re-do without backwards compatibility concerns.

I also increased the test coverage to address our concerns not knowing how it would work with the lazy nature of rspec on let, and before/after/around hooks.

[codecov-io]

Codecov Report

Merging #1381 (b233d53) into master (b06b84f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1381   +/-   ##
=======================================
  Coverage   98.13%   98.14%           
=======================================
  Files         755      754    -1     
  Lines       35885    35946   +61     
=======================================
+ Hits        35217    35279   +62     
+ Misses        668      667    -1     

Impacted Files	Coverage Δ
lib/ddtrace/contrib/rspec/ext.rb	100.00% <ø> (ø)
lib/ddtrace/contrib/rspec/integration.rb	100.00% <ø> (ø)
lib/ddtrace/contrib/rspec/patcher.rb	100.00% <ø> (ø)
spec/ddtrace/contrib/rspec/patcher_spec.rb	100.00% <ø> (ø)
lib/ddtrace/contrib/rspec/example.rb	94.59% <100.00%> (+1.49%)	⬆️
spec/ddtrace/contrib/rspec/instrumentation_spec.rb	100.00% <100.00%> (ø)
...b/action_pack/action_controller/instrumentation.rb	94.28% <0.00%> (-1.43%)	⬇️

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[juan-fernandez]

LGTM. Not like my Ruby skills are off the charts 😆