statsd: add container-based config discovery support

[vitkyrka]

What does this PR do?

Adds container-based config discovery support to statsd.

Docker-compose change: the fixture builds a custom image from statsd/statsd:${STATSD_VERSION} with no image: tag, so Compose named the built image <project>-statsd, which never matched the statsd ad_identifiers value; added an explicit image: statsd/statsd:${STATSD_VERSION} tag so the locally-built image keeps the same name a real deployment would use. Also added a ${DD_LOG_1:-/dev/null} fallback (target is a file, so the fallback is /dev/null) since assert_all_discovery_candidates_stable's docker compose ps subprocess call doesn't inherit docker_run's env_vars=.

Port selection rationale: Port 8126 is StatsD's official documented default management/admin TCP interface port (mgmt_port in config.js), which is exactly the port the check's StatsCheck.check() connects to over a raw TCP socket to issue health/stats/counters/gauges/timers commands.

Endpoint shape: StatsD talks raw TCP, not HTTP, so spec.yaml uses the manual from_ports strategy with separate host/port candidate fields instead of the discovery/openmetrics_from_ports template.

Exact-tag-match test helper: the existing test_e2e's shared assert_metrics_covered() helper asserts an exact host:/port: tag pair that the check derives from its own connection parameters. Under discovery the Agent connects directly to the container's own internal host/port (unknown ahead of time), so test_e2e_discovery asserts metric/service-check presence without those tags instead of reusing assert_metrics_covered() verbatim.

Motivation

https://datadoghq.atlassian.net/browse/DSCVR-610

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add qa/required if this PR needs QA validation, or qa/skip-qa if it does not. Exactly one of the two is required.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[datadog-prod-us1-5]

 

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 50.00%
• Overall Coverage: 82.24%

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[dd-octo-sts]

Validation Report

All 21 validations passed.

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Validation	Description	Status
agent-reqs	Verify check versions match the Agent requirements file	✅
ci	Validate CI configuration and code coverage settings	✅
codeowners	Validate every integration has a CODEOWNERS entry	✅
config	Validate default configuration files against spec.yaml	✅
dep	Verify dependency pins are consistent and Agent-compatible	✅
http	Validate integrations use the HTTP wrapper correctly	✅
imports	Validate check imports do not use deprecated modules	✅
integration-style	Validate check code style conventions	✅
jmx-metrics	Validate JMX metrics definition files and config	✅
labeler	Validate PR labeler config matches integration directories	✅
legacy-signature	Validate no integration uses the legacy Agent check signature	✅
license-headers	Validate Python files have proper license headers	✅
licenses	Validate third-party license attribution list	✅
metadata	Validate metadata.csv metric definitions	✅
models	Validate configuration data models match spec.yaml	✅
openmetrics	Validate OpenMetrics integrations disable the metric limit	✅
package	Validate Python package metadata and naming	✅
qa-label	Validate the pull request declares whether it needs QA for the next Agent release	✅
readmes	Validate README files have required sections	✅
saved-views	Validate saved view JSON file structure and fields	✅
version	Validate version consistency between package and changelog	✅

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