feat: OTel JVM semconv compliance (dual-emit jvm.* alongside argus_*)

[rlaope]

Summary

Workstream W1 — OTel JVM semantic-convention compliance.

Argus now speaks the OpenTelemetry JVM runtime semantic conventions. A new single
source-of-truth mapping layer (SemconvMetrics) maps Argus GC/heap/thread/class/cpu
metrics to the standard OTel names, and both the Prometheus collector and the OTLP
exporter draw their names, units, and types from that one table.

Standard metrics covered: jvm.gc.duration, jvm.memory.used, jvm.memory.committed,
jvm.memory.used_after_last_gc, jvm.thread.count, jvm.class.count, jvm.cpu.time,
jvm.cpu.recent_utilization — with UCUM units (s, By, 1, {thread}, {class})
and the standard attributes (jvm.gc.name, jvm.gc.action, jvm.memory.pool.name).
In Prometheus exposition the dots become underscores (jvm_gc_duration_seconds, etc.).

Compatibility

• The legacy argus_* series are still emitted by default, gated behind a new flag
  argus.metrics.legacyNames (default true). Existing dashboards keep working;
  set argus.metrics.legacyNames=false to emit only the standard jvm_* names.
• Argus-unique metrics keep the argus.* namespace and are emitted regardless of
  the flag — they are differentiators, not "legacy" duplicates. Examples:
  argus_gc_leak_confidence, argus_gc_leak_suspected, argus_gc_overhead_ratio,
  GC allocation/promotion rates, carrier-thread skew, argus_metaspace_reserved_bytes,
  virtual-thread start/end/pinning counters, profiling samples.

OTLP resource attributes

OtlpMetricsExporter (via OtlpJsonBuilder) now stamps the full OTel resource set:
service.name, service.namespace (K8s namespace via Downward API), service.instance.id
(pod name / hostname), telemetry.sdk.name, telemetry.sdk.language=java,
telemetry.sdk.version. The OTLP payload also emits the semconv metric names alongside
the legacy names.

Acceptance criteria and verification

1. Semconv mapping layer (single source of truth): SemconvMetrics holds the table
   (OTel name, Prometheus name, unit, type, description, attributes). Verified by
   SemconvMetricsComplianceTest.mapping_table_units_are_correct and
   mapping_table_declares_required_attributes.
2. Dual-emit + legacy flag: Prometheus emits jvm_* alongside argus_* by default;
   argus.metrics.legacyNames=false drops the argus_* duplicates. Verified by
   emits_standard_jvm_series_alongside_legacy_by_default and
   disabling_legacy_names_drops_argus_duplicates_but_keeps_jvm_and_unique.
3. OTLP resource attributes + semconv names: see OtlpJsonBuilder.appendResource
   and the semconv emissions in each metric section.
4. /prometheus stays valid: every series has HELP/TYPE and there are no duplicate
   series. Verified by every_series_has_help_and_type_and_no_duplicate_series.
5. Mapping table covers every exported standard metric, units correct: verified by
   emitted_jvm_names_match_the_mapping_table and mapping_table_units_are_correct.
6. Argus-unique metrics not renamed, emitted regardless of the flag: verified by
   argus_unique_metrics_emitted_regardless_of_legacy_flag.

Deviations

• jvm.cpu.time (a monotonic process-CPU-seconds counter) has no JFR-derived source in
  Argus today — only the recent-utilization ratio is available. It is kept in the mapping
  table as the documented contract but is not exported yet. The mapping table is the
  superset/contract, so the "every exported metric is in the table" assertion still holds.
• The jvm.gc.duration histogram is exported as the existing aggregate; it is not yet
  split per jvm.gc.name/jvm.gc.action (Argus tracks only an aggregate pause histogram).
  The per-collector/per-cause breakdown remains available as the Argus-unique
  argus_gc_pause_breakdown_seconds_total series.

Test / build notes

./gradlew :argus-server:test --tests "io.argus.server.metrics.*" and
:argus-core:test are green. Two unrelated, pre-existing failures on master are out of
scope for W1 and reproduce with this branch's changes stashed:
CorrelationAnalyzerTracePauseTest (uses hardcoded 2026-05-28 timestamps that the 5-minute
retention prunes) and a Gradle implicit-dependency validation error on :argus-server:jar
vs :argus-diagnostics:jar.

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