Add input_truncated metric to appsec.waf.requests telemetry

[y9v]

What does this PR do?
This PR adds input_truncated metric to appsec.waf.requests telemetry.

Motivation:
This change is extracted to a separate PR from #4839, since it required changes to libddwaf-rb gem.

Change log entry
None. This change is internal.

Additional Notes:
Follow up for #4839
Related libddwaf-rb PR: DataDog/libddwaf-rb#91

This PR is in draft mode until changes to libddwaf-rb are merged and released.

How to test the change?
CI and manual testing.

[datadog-official]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 98.41%
• Total Coverage: 95.19% (+0.01%)

View detailed report

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-09-17 14:40:53

Comparing candidate commit 3c3e080 in PR branch add-input-truncation-metric-to-waf-requests-telemetry with baseline commit 2f7a9d3 in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:profiling - intern_all 1000 repeated strings

• 🟩 throughput [+1224.876op/s; +1288.036op/s] or [+5.368%; +5.645%]

[Strech]

👍🏼 LGTM