Add NatsPartitioning and audit streaming SLIs

What does this MR do and why?

Add NatsPartitioning and Gitlab::Metrics::AuditEventStreamingSlis for audit event streaming

• NatsPartitioning: deterministic mapping of a top-level group ID to one of PARTITION_COUNT subjects (group_id % PARTITION_COUNT), giving stable per-group FIFO ordering and bounded subject cardinality. Instance-scoped events (no root group) get a dedicated audit_events.streaming.instance subject and durable so they are isolated from tenant backlogs rather than sharing a numeric partition.
• Gitlab::Metrics::AuditEventStreamingSlis: instrumentation for the pipeline. Fallback to Sidekiq is a plain degradation counter (fallbacks/attempts), not an error-rate SLI, so a degraded NATS path does not drain the Compliance error budget when delivery still succeeded via Sidekiq. Dispatch failures are a real error-rate SLI; consumer lag is an apdex SLI plus a raw histogram for quantiles. The SLIs pre-create every label combination at boot.
• Document the metrics in gitlab_metrics.md and application_slis.

These are pure helpers with no callers yet will be wired up in later changes.

Part of epic &17582

Related to #604455

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