Fix invalid metric unit strings per OTel UCUM spec

[esosaoh]

Fixes #1273.

Replaces "${description}" unit strings with "{description}" across four monitoring classes. The $ prefix is invalid in this context — UCUM annotations use plain curly braces. Other instruments in this codebase (SCMEventMonitoringInitializer, MonitoringCloudListener) already use the correct form.

Testing done

String literal changes to metric unit registration. No existing tests assert on instrument unit strings (test suite validates metric names and values, not units). Verified:

• Build passes: ./mvnw clean install
• Code formatting compliant: ./mvnw spotless:check
• Changed lines executed: Build output confirms all four modified classes are compiled and indexed as Jenkins extensions:
  • io.jenkins.plugins.opentelemetry.init.JenkinsExecutorMonitoringInitializer indexed under hudson.Extension
  • io.jenkins.plugins.opentelemetry.init.PluginMonitoringInitializer indexed under hudson.Extension
  • io.jenkins.plugins.opentelemetry.queue.MonitoringQueueListener indexed under hudson.Extension
  • io.jenkins.plugins.opentelemetry.security.AuditingSecurityListener indexed under hudson.Extension
• Integration tests exercise changed code: Test suite includes JenkinsOtelPluginIntegrationTest and JenkinsOtelPluginFreestyleIntegrationTest which instantiate Jenkins with the plugin loaded, triggering @PostConstruct methods that register these metrics

No new tests added, existing test coverage validates that metrics are registered and emit values correctly. The test suite does not assert on instrument unit strings (only metric names and values), which is appropriate since unit strings are metadata consumed by observability backends, not Jenkins runtime behavior.

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