feat(metrics): expose dropped messages count as the droppedMsgs Prometheus counter

[dmytro-landiak]

Pull Request description

Resolves #303.

Exposes the cumulative dropped-message count as a Micrometer counter droppedMsgs on the existing /actuator/prometheus endpoint, so operators can alert on message drops directly with Prometheus.

What changed

• Added a dedicated TbMessageStatsReportClient#reportDroppedMsgs(int count) method.
• TbMessageStatsReportClientImpl now injects MeterRegistry, registers a single Counter named droppedMsgs (no tags), and increments it on every dropped message. The counter is registered in @PostConstruct init() before the historical-data-report.enabled early-exit, so the metric is always present and scrapeable — even when historical persistence is disabled.
• Migrated all 14 existing reportStats(DROPPED_MSGS[, n]) call sites (11 files) to reportDroppedMsgs(...). No call site uses the generic reportStats path for dropped messages anymore.

Backward compatibility

• The existing in-memory AtomicLong → PostgreSQL historical pipeline is unchanged and still gated by historical-data-report.enabled, so existing historical droppedMsgs dashboards / time series are unaffected. No schema change, no new dependency.

Sample scrape (/actuator/prometheus):

# HELP droppedMsgs_total
# TYPE droppedMsgs_total counter
droppedMsgs_total 0.0

Documentation

• The Prometheus / monitoring documentation should be updated to mention the new droppedMsgs_total counter exposed on /actuator/prometheus.

General checklist

• You have reviewed the guidelines document.
• Labels that classify your pull request have been added.
• The milestone is specified and corresponds to fix version.
• Description references specific issue ([Feature Request] Expose dropped messages count as a Prometheus metric #303).
• Description contains human-readable scope of changes.
• Description contains brief notes about what needs to be added to the documentation.
• No merge conflicts, commented blocks of code, code formatting issues.
• Changes are backward compatible or upgrade script is provided.

Front-End feature checklist

N/A — back-end only change (no front-end / UI changes).

• Screenshots with affected component(s) are added. The best option is to provide 2 screens: before and after changes;
• If you change the widget or other API, ensure it is backward-compatible or upgrade script is present.

Back-End feature checklist

• Added corresponding unit and/or integration test(s). Provide written explanation in the PR description if you have failed to add tests. — 3 new unit tests in TbMessageStatsReportClientImplTest (basic increment, batched count, and counter-works-when-historical-reporting-disabled); BasicDownLinkProcessorImplTest verifications updated; full mvn -T 1C clean install is green, including the Spring-context RateLimitsIntegrationTestCase.
• If new dependency was added: the dependency tree is checked for conflicts. — No new dependency added; Micrometer / micrometer-registry-prometheus were already on the classpath.