Add StatsDCounterKey/StatsDCountReporter glue for Accumulator

[dougqh]

What Does This Do

Adds StatsDCounterKey and StatsDCountReporter in products/metrics/metrics-api: reusable glue that lets an enum used as an Accumulator key declare its own dogstatsd metric name at the declaration site, and drains an Accumulator.accumulateAndReset(...) result to a StatsDClient in one call, skipping zero-delta counters.

This is groundwork only -- no caller is wired in yet. The natural first caller is TracerHealthMetrics (currently ~49 individual LongAdder fields, each diffed by hand against a fixed-size previousCounts array), but that migration is scoped to a separate follow-up PR.

Motivation

Accumulator (#12351) has no consumer yet. Before wiring one in, this PR builds the small reusable piece that makes adoption mechanical: one interface for an enum to name its own metric, one static helper to report a drained counter array. Keeping this separate from the TracerHealthMetrics migration keeps that follow-up PR focused on the migration itself rather than also inventing this glue inline.

This is explicitly a consolidation move: the codebase already has several ad hoc ways to get a counter's value to a metrics sink (CoreCounter/StatsMetrics.TaggedCounter, the MetricCollector SPI, CiVisibilityCountMetric's tag-combination encoding). None of them fit Accumulator's enum-ordinal, tag-less shape, so this adds the smallest new piece rather than reusing a mismatched one -- see the design note in StatsDCounterKey's Javadoc for the intended usage shape.

Additional Notes

• products/metrics/metrics-api had no test dependencies before this PR; added testImplementation(libs.bundles.junit5) (matches the pattern in sibling modules metrics-lib, feature-flagging-api).
• Test plan:
  • ./gradlew :products:metrics:metrics-api:test --tests "datadog.metrics.api.statsd.StatsDCountReporterTest" -- all pass
  • ./gradlew :products:metrics:metrics-api:spotlessCheck -- clean
  • /techdebt and /perf-review run over branch changes -- no findings on either pass

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: APMLP-1779

[datadog-official]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 58.80% (-0.06%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 264a2ef | Docs | View more details | Give us feedback!}

[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	13.90 s	13.95 s	[-0.9%; +0.2%] (no difference)
startup:insecure-bank:tracing:Agent	12.91 s	13.06 s	[-2.0%; -0.3%] (maybe better)
startup:petclinic:appsec:Agent	16.90 s	16.76 s	[-0.3%; +1.9%] (no difference)
startup:petclinic:iast:Agent	16.91 s	16.90 s	[-0.8%; +0.9%] (no difference)
startup:petclinic:profiling:Agent	16.70 s	16.72 s	[-1.3%; +1.0%] (no difference)
startup:petclinic:sca:Agent	16.62 s	16.38 s	[+0.6%; +2.4%] (maybe worse)
startup:petclinic:tracing:Agent	16.05 s	15.96 s	[-0.2%; +1.3%] (no difference)

Commit: 264a2ef4 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.