Improve controller instrumentation and deprecate option exception_controller

[TonyCTHsu]

Why?

This PR addresses issues (#2709 , #2611 , #2567) we've had for instrumenting an exception handling controller.

1. The trace resource being overwritten by the exception handling controller, make it hard understand the origin of error.
2. Configuration for exception_controller with constant does not work properly with zeitwerk autoloading.

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What?

After careful examination of the problem, I have concluded that the current behaviour of option exception_controller is bringing more harm than benefit, because of the fundamental problem to answer the question about what is the strategy to decide whether to overwrite trace resource?

🚫 It should not be made by based on whether the current controller is handling the request being defined as an exceptional controller, since a request could be sent to the path directly (such case: overwrite trace resource).
✅ The decision should be made by whether an exception being redirected by ActionDispatch::ShowException.

The changes are made to depend on the default behaviour for ActionDispatch::ShowException and make the option exception_controller obsolete. We decided to deprecate the option and no longer required to solve the problem with zeitwerk autoloading.

We recommend our users to remove exception_controller option from the configuration.

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Snapshot of changes

[codecov-commenter]

Codecov Report

Merging #2726 (145d81f) into master (3c4c4ba) will increase coverage by 0.01%.
The diff coverage is 98.59%.

@@            Coverage Diff             @@
##           master    #2726      +/-   ##
==========================================
+ Coverage   98.06%   98.07%   +0.01%     
==========================================
  Files        1210     1215       +5     
  Lines       66456    67025     +569     
  Branches     2972     2994      +22     
==========================================
+ Hits        65172    65738     +566     
- Misses       1284     1287       +3     

Impacted Files	Coverage Δ
spec/datadog/core/telemetry/collector_spec.rb	100.00% <ø> (ø)
spec/datadog/core/telemetry/v1/app_event_spec.rb	100.00% <ø> (ø)
...atadog/core/telemetry/v1/telemetry_request_spec.rb	100.00% <ø> (ø)
lib/datadog/core/remote/component.rb	88.00% <66.66%> (-7.46%)	⬇️
...og/tracing/contrib/rails/configuration/settings.rb	97.43% <80.00%> (+0.29%)	⬆️
lib/datadog/core/utils/hash.rb	84.61% <84.61%> (ø)
...cing/contrib/action_pack/configuration/settings.rb	96.00% <85.71%> (-4.00%)	⬇️
.../datadog/appsec/contrib/rack/request_middleware.rb	95.29% <90.90%> (-0.81%)	⬇️
lib/datadog/core/remote/client.rb	98.16% <93.10%> (-1.84%)	⬇️
lib/datadog/appsec/processor.rb	95.93% <95.65%> (+0.65%)	⬆️
... and 28 more

... and 4 files with indirect coverage changes

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