feat: Expose version information as a Prometheus metric

[jason-p-pickering]

Adds VersionMetrics, which registers a dhis2_version_info gauge on the /api/metrics scrape endpoint, exposing the running instance's version, revision, and build time as labels (Prometheus "info" pattern —value is always 1, data lives in labels). Lets ops dashboards/alerts track which build is deployed alongside other DHIS2 metrics, without having to separately poll system/info.

Values are read once at startup from SystemService#getSystemInfo() (baked in at build time), so no runtime overhead. No-ops cleanly if no MeterRegistry is present.

Sample output:

# HELP dhis2_version_info DHIS2 build version, revision and build time, exposed as labels; value is always 1
# TYPE dhis2_version_info gauge
dhis2_version_info{build_time="2026-09-03T08:04:32Z",revision="86e3491",version="2.44-SNAPSHOT"} 1

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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