Emit untagged telemetry when OpenMetrics reaches its metric limit

[nubtron]

What does this PR do?

Emits the untagged openmetrics.max_returned_metrics_reached Agent telemetry counter once per OpenMetrics check run when the existing metric limiter reaches max_returned_metrics.

The producer checks the limiter's structured reached_limit state before reset, then uses the existing four-argument datadog_agent.emit_agent_telemetry API. OpenMetrics v1 and v2 opt in; generic AgentCheck limiters do not. Emission is best-effort and cannot alter warning, drop, or reset behavior.

The metric intentionally carries no tags, including no check_name.

Motivation

AI-7012 asks for a fleet-wide COAT signal for silent OpenMetrics drops after max_returned_metrics is reached. An untagged first version provides that signal while avoiding warning-text parsing, new rtloader callbacks, and a generic labeled telemetry bridge.

Validation included focused metric-limiter tests covering v1/v2 opt-in, once-per-run emission, generic-check exclusion, old-Agent compatibility, bridge failure behavior, and debug metric contexts. The complete datadog_checks_base format/lint target also passed.

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	✅

View full run

[datadog-datadog-us1-prod]

 

🎉 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: 88.45% (+0.11%)

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