Observability.
Emits the per-request causal trace (shapes, ids, field names, never values,
docs/05) and assembles the /debug/explain/{request_id} document for LLM
consumption. Read-only: it never mutates routing or cluster state
(docs/decisions/005).
The [RequestTrace] is shape-only by construction, its setters accept
only id newtypes, compile-time labels, and sizes, so there is no API path by
which a tenant value or secret can reach telemetry (docs/05 §7). The
[ExplainStore] retains the most recent explanations for the debug endpoint.
Directive evaluation (runtime verbosity control) lives in osproxy-control;
[resource_spans] encodes that same shape-only trace as an OTLP/HTTP JSON
payload for export (the wire emission is the I/O layer's job, M7).