Emit telemetry when OpenMetrics reaches its metric limit

[nubtron]

What does this PR do?

• Add an optional first-limit-reached callback to the metric limiter while preserving its existing warning, counting, de-duplication, and drop behavior.
• Opt OpenMetrics v1 and v2 checks into a best-effort openmetrics.max_returned_metrics_reached Agent telemetry counter, emitted once per limiter warning with only the canonical check_name label.
• Feature-detect the labeled Agent bridge so newer datadog_checks_base remains compatible with older Agents, and keep the signal independent from debug_metrics.metric_contexts.

Validation:

• ddev --no-interactive test datadog_checks_base -- tests/base/utils/test_utils.py -k limiter
• ddev --no-interactive test datadog_checks_base -- tests/base/checks/test_agent_check.py -k 'metric_limit or debug_metrics or telemetry'
• ddev --no-interactive test -fs datadog_checks_base
• ddev --no-interactive test --lint datadog_checks_base
• Combined custom-Agent smoke tests for OpenMetrics v1 and v2, confirming one warning, raw cumulative value 1, and only check_name=openmetrics in the filtered payload.

Motivation

OpenMetrics drops metric contexts beyond max_returned_metrics for the remainder of a check run. Today this is visible only through a one-per-run warning. Emitting a low-cardinality structured counter at the same transition makes this degradation observable without exposing endpoint or instance identity.

Companion Agent bridge and Agent Telemetry profile: DataDog/datadog-agent#53751

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

[dd-octo-sts]

Validation Report

All 21 validations passed.

Show details

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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[datadog-datadog-us1-prod]

   

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 3 Pipeline jobs failed

PR All | test / j06ca546 / SNMP     

PR All | test / j92d42de / ArgoCD     

PR All | test / j71c1491 / Apache NiFi     

🧪 3 Tests failed in 1 job

PR All | run  

test_e2e_openmetrics_v1 from test_e2e.py  

Needed exactly 2 candidates for 'argocd.app_controller.openmetrics.health', got 0
Expected:
        ServiceCheckStub(check_id=None, name='argocd.app_controller.openmetrics.health', status=0, tags=None, hostname=None, message=None)
Difference to closest:
        Expected name: argocd.app_controller.openmetrics.health
        Found argocd.appset_controller.openmetrics.health

Similar submitted:
Score   Most similar
0.97    ServiceCheckStub(check_id='argocd:fe8f69051fb4528', name='argocd.appset_controller.openmetrics.health', status=0, tags=['endpoint:http://10.1.0.221:45719/metrics'], hostname='runnervm76tkm', message='')
...

test_e2e from test_e2e.py  

[s6-init] making user provided files available at /var/run/s6/etc...exited 0.
[s6-init] ensuring user provided files have correct perms...exited 0.
[fix-attrs.d] applying ownership & permissions fixes...
[fix-attrs.d] done.
[cont-init.d] executing container initialization scripts...
[cont-init.d] 01-check-apikey.sh: executing... 
[cont-init.d] 01-check-apikey.sh: exited 0.
[cont-init.d] 50-ci.sh: executing... 
[cont-init.d] 50-ci.sh: exited 0.
[cont-init.d] 50-ecs-managed.sh: executing... 
...

View all failed tests

ℹ️ Info

No other issues found (see more)

❄️ No new flaky tests detected

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 88.40% (+0.09%)

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