chore: enable runtime metrics for test apps

[felixge]

The agent was not accepting custom metrics yet. Update the env variables ot enable dogstatsd.

What does this PR do?

Motivation

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.

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[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-12 14:29:50

Comparing candidate commit 61f2c77 in PR branch felix.geisendoerfer/chore-enable-runtime-metrics-for-test-apps with baseline commit ef22cc7 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 3 metrics, 0 unstable metrics.