feat(appsec/proxy): add metrics

[e-n-0]

What does this PR do?

This PR implement the collection of metrics for appsec proxies:

• collect environment variables value (and origin: default or custom set)
• collect message processor configuration at the first request received
• collect body size of request/response body messages processed

As the telemetry package is internal and the environment variables are set inside the contribs, the method TelemetryRegisterAppConfig and the telemetry origin have been exposed inside the instrumentation package.*

Related

PR that adds the metrics to dd-go.

Motivation

More visibility into the usage of the product.

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.
• New code is free of linting errors. You can check this by running ./scripts/lint.sh locally.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-10-28 13:47:43

Comparing candidate commit 82e25fa in PR branch flavien/proxies/metrics with baseline commit 399a58b in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 82e25fa | Docs | Datadog PR Page | Was this helpful? Give us feedback!}

[e-n-0]

Waiting for the changes merge in dd-go to update the metric values in this PR.

[e-n-0]

Reopened the PR as PR in dd-go merged.

[eliottness]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Breezy!

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[e-n-0]

/merge

[dd-devflow-routing-codex]

View all feedbacks in Devflow UI.

2025-10-29 11:47:15 UTC ℹ️ Start processing command /merge

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2025-10-29 11:47:20 UTC ℹ️ MergeQueue: pull request added to the queue

The expected merge time in main is approximately 18m (p90).

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2025-10-29 12:02:12 UTC ℹ️ MergeQueue: This merge request was merged