flink: add container-based config discovery support

[vitkyrka]

What does this PR do?

Adds container-based config discovery support to flink.

This only supports discovering the OpenMetrics endpoint, not the legacy Datadog HTTP Reporter.

• Port selection rationale: Port 9249 is the documented default port for Flink's flink-metrics-prometheus reporter.
• AD identifier: flink, the official Docker Hub image.
• Both roles are covered: Since jobmanager and taskmanager share one image and both expose the Prometheus reporter on the same internal port (9249), Autodiscovery independently discovers and configures one instance per container — no per-role config field is needed to disambiguate them. test_e2e_discovery passes discovery_min_instances=2 and asserts both jobmanager.* and taskmanager.* core metrics to verify this.
• Log-pattern narrowing: Probing the RPC candidate (port 6123) with an HTTP client makes Flink's Pekko/Netty stack log a TooLongFrameException ... discarded warning (not an error-level match, so no test change needed for that). Separately, Flink always logs a one-off benign startup notice ("Hadoop FS is not available ...: NoClassDefFoundError") in the vanilla flink image, which does match the default error stability pattern. test_e2e_discovery_all_candidates narrows that one pattern with a negative lookbehind ((?<!NoClassDefFound)error) so only that known-benign substring is excluded, leaving real error detection intact for everything else.

Motivation

https://datadoghq.atlassian.net/browse/DSCVR-538

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-datadog-prod-us1]

 

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog auto-retried 1 job - 1 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 100.00%

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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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