[2748109647]

@fstab

Fixes #2632.

StateSet.Builder inherited the generic unit(...) setter, so it accepted a non-null unit while building and only failed later during collect(). Override the setter to reject non-null units immediately, consistent with Info.Builder, and add a regression test.

Validation:

• mvnw.cmd test -pl prometheus-metrics-core -am -Dtest=StateSetTest -Dsurefire.failIfNoSpecifiedTests=false -Dcoverage.skip=true — 5 tests passed; reactor build succeeded.
• mvnw.cmd install -DskipTests -DskipITs -Dcoverage.skip=true — all 56 modules built successfully.
• mise run lint:fix applied formatting. Checkstyle could not run locally because its downloaded Windows executable reported an incompatible binary.
• Docker/Testcontainers integration checks were attempted with Docker available, but the configured docker.mirrors.ustc.edu.cn registry mirror did not resolve when pulling testcontainers/ryuk:0.12.0; those checks remain unverified.

[zeitlinger]

Run mise run api-diff and commit the generated update

[github-actions]

⚠️ API changes detected — maintainer review required

This PR modifies the published API diff for the following module(s):

• prometheus-metrics-core

Please review the changes in docs/apidiffs/current_vs_latest/ carefully before approving.

[2748109647]

Done in commit 60a51f1 (60a51f1). I reviewed the generated diff; it records the new public StateSet.Builder.unit(Unit) method introduced by this PR.

The documented Maven API-diff reactor completed successfully across all 30 modules with -Dmaven.test.skip=true, and regenerated the API-diff output. The exact mise run api-diff task could not complete in this environment: mise reported a Java 25 lock/version mismatch and stalled while provisioning Renovate's npm dependencies. The fallback skipped test-source compilation and all tests. On the pushed commit, the API-change detector and DCO checks both pass; the remaining checks are still pending.