[APMAPI-1356] Fix Telemetry `origin` field for `app-started` event

[vpellan]

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What does this PR do?

This PR fixes the origin field in app-started event.

Motivation:

I wanted to add support for local_stable_config and fleet_stable_config origin, but I realised that it is currently almost always hardcoded to code.

Change log entry

None.

Additional Notes:

How to test the change?

[datadog-datadog-prod-us1[bot]]

Datadog Report

Branch report: vpellan/fix-telemetry-origin-field Commit report: e5f6ec5 Test service: dd-trace-rb

:white_check_mark: 0 Failed, 21787 Passed, 1296 Skipped, 4m 56.64s Total Time

[pr-commenter[bot]]

Benchmarks

Benchmark execution time: 2025-06-30 10:27:09

Comparing candidate commit 831276fa49d38147eb9800bba9b138544d95c361 in PR branch vpellan/fix-telemetry-origin-field with baseline commit 9a889061676d5d257de149ec68b56805aab390f3 in branch master.

Found 0 performance improvements and 1 performance regressions! Performance is the same for 42 metrics, 6 unstable metrics.

scenario:profiling - Allocations ()

• 🟥 throughput [-231101.212op/s; -217210.695op/s] or [-6.949%; -6.532%]

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.93617% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.55%. Comparing base (9a88906) to head (831276f).

Files with missing lines	Patch %	Lines
lib/datadog/core/telemetry/event/app_started.rb	97.67%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4764      +/-   ##
==========================================
- Coverage   97.56%   97.55%   -0.01%     
==========================================
  Files        1486     1486              
  Lines       88716    88758      +42     
  Branches     4599     4603       +4     
==========================================
+ Hits        86552    86587      +35     
- Misses       2164     2171       +7     

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