DEBUG-2334 add new_record flag to ActiveRecord model custom serializer

[p-datadog]

What does this PR do?
Dynamic Instrumentation ships with a custom serializer for ActiveRecord models which lifts the interesting fields (i.e. attributes) to the top level of the serialized object.

This PR adds the new_record? flag to the serialized object.

Motivation:
The current rendering of AR models in dynamic instrumentation UI is somewhat "weird" because the attributes are nested under the "attributes" field, but there are no other fields. It's confusing why the attributes aren't just rendered on the top level.

Adding the new_record flag, in my opinion, improves the UI to where it looks more polished and doesn't look like something is wrong with it.

Change log entry
None (part of initial DI release)

Additional Notes:

How to test the change?

Unit tests are included

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-11-26 17:59:27

Comparing candidate commit 35b0afd in PR branch di-ar-new-record with baseline commit ce4393e in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 30 metrics, 2 unstable metrics.

scenario:profiler - sample timeline=false

• 🟩 throughput [+0.588op/s; +0.597op/s] or [+9.887%; +10.041%]

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.78%. Comparing base (ce4393e) to head (35b0afd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4165   +/-   ##
=======================================
  Coverage   97.78%   97.78%           
=======================================
  Files        1353     1353           
  Lines       81817    81818    +1     
  Branches     4145     4145           
=======================================
+ Hits        80001    80008    +7     
+ Misses       1816     1810    -6     

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