Fix http.route tag for rack apps mounted in Rails

[y9v]

What does this PR do?
This PR removes the behavior that used the request path as a fallback when computing the route for Rack applications mounted inside Rails.

Motivation:
The current approach to setting the http.route tag for mounted Rack applications causes issues for the API Catalog. Specifically, when a mounted Rack application includes parameters in its request path, it can lead to an unbounded number of reported routes.

This issue can occur in the following cases:

• Mounted applications such as the Sidekiq web UI, which is a Rack app with a simple router.
• Mounted Sinatra and Grape applications, when the Datadog tracer is configured to instrument Rails but not Sinatra or Grape directly.

Before the fix route for an embedded Rack app would look like /path/for/mounted/app/users/1, after the fix it would be /path/for/mounted/app.

Change log entry
Yes. Fix http.route tag for Rack applications mounted inside Rails.

Additional Notes:
APPSEC-59663

How to test the change?
CI and app generator.

[Strech]

🔥 Amazing! Well done!

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 100.00%
• Total Coverage: 98.56% (+0.01%)

View detailed report

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-10-21 11:56:56

Comparing candidate commit 433b9c9 in PR branch fix-http-route-tagging-for-mounted-rack-apps with baseline commit f8dfc57 in branch master.

Found 1 performance improvements and 1 performance regressions! Performance is the same for 42 metrics, 2 unstable metrics.

scenario:error - without error tracking, with_error=false

• 🟥 throughput [-678.981op/s; -630.264op/s] or [-5.507%; -5.112%]

scenario:profiling - Allocations (baseline)

• 🟩 throughput [+290078.822op/s; +303636.330op/s] or [+5.806%; +6.078%]

[vpellan]

LGTM!