feat(observability): per-call OpenTelemetry spans (closes #9 — final FleetQ ask)

[escapeboy]

Sprint 0.4-S5 — closes #9 (FleetQ P2, the LAST ask)

Final FleetQ implementer-feedback ask. Closes the letter completely — ALL 9 P1/P2 asks shipped (#3, #5, #6, #7, #8, #9 + the two P0s in v0.2.0). 6 sprints in a row driven by FleetQ signal.

Surface

```bash
export OTEL_EXPORTER_OTLP_ENDPOINT=http://collector.internal:4318
export OTEL_SERVICE_NAME=my-agent # optional, defaults to "boruna"
boruna run app.ax --policy allow-all --live
```

Each capability call emits a `boruna.cap` span:

```
boruna.cap {cap.name="net.fetch", bytes_in=247, bytes_out=1834, cap.budget_remaining=42} 142ms
boruna.cap {cap.name="llm.call", bytes_in=890, bytes_out=2103, cap.budget_remaining=8} 7.4s
boruna.cap {cap.name="fs.read", bytes_in=64, bytes_out=12834} 8ms
```

What's in this PR

Always-on tracing (zero-cost when no subscriber)

`tracing = "0.1"` is now a non-optional dep on `boruna-vm`. `CapabilityGateway::call` wraps the call body in a `tracing::info_span!` named `boruna.cap`. When no subscriber is installed (the default), span macros are essentially no-ops — single atomic check, a few stack-allocated structs, no allocations.

Span attributes:

Field	Type	Notes
`cap.name`	string	e.g. `"net.fetch"`
`bytes_in`	u64	sum of UTF-8 string content reachable from args (recurses through containers)
`bytes_out`	u64	UTF-8 string content of result (recurses through Record/Enum/etc)
`cap.budget_remaining`	u64	post-call quota when a budget rule applies
`error.kind`	string	only on failure: `denied` / `budget_exceeded` / `runtime_error`

`telemetry` Cargo feature

Adds `opentelemetry 0.27` + `opentelemetry_sdk 0.27` (rt-tokio) + `opentelemetry-otlp 0.27` (http-proto + reqwest-client, default-features off) + `tracing-opentelemetry 0.28` + `tracing-subscriber 0.3` + `tokio` (workspace, optional).

New module `boruna_vm::telemetry` with `init() -> TelemetryHandle`:

• Endpoint env var unset → `Disabled` no-op handle (Boruna behaves identically to a non-telemetry build).
• Endpoint set → installs OTLP-over-HTTP exporter into the global tracing subscriber registry.
• `TelemetryHandle::Drop` calls `force_flush()`.

CLI integration

New `telemetry` feature on `boruna-cli`. When built with `--features telemetry`, `main` starts a tokio runtime, calls `init_telemetry()` BEFORE parsing CLI args, holds the handle for the binary lifetime, and on shutdown drops the handle THEN drains the runtime with a 5-second timeout (so in-flight OTel HTTP POSTs complete instead of being killed by `process::exit`).

Determinism contract (per ADR 001)

Spans are operational metadata only. Their content (durations, byte counts) is never fed into an `EventLog`, `AuditLog`, or `EvidenceBundle`. A replayed run produces identical replay state but may produce different span durations on a faster/slower host — by design. Documented in `CapabilityGateway::call` doc comment + `boruna_vm::telemetry` module doc.

Tests

• 4 new VM tests using a per-test scoped tracing subscriber with proper Id-keyed span matching:
  • `test_capability_call_emits_boruna_cap_span_with_attributes`
  • `test_capability_call_records_bytes_out_for_string_returning_handler`
  • `test_capability_call_records_error_kind_denied`
  • `test_capability_call_records_error_kind_budget_exceeded`
• 1 telemetry-module test (Disabled handle Drop is a clean no-op)
• All 591+ existing workspace tests pass
• `cargo clippy --workspace -- -D warnings` clean (with and without `--features boruna-vm/telemetry`)
• `cargo fmt --all -- --check` clean

Review

`ce-correctness-reviewer` surfaced 5 HIGH findings. All addressed before commit:

#	Finding	Fix
1	Env-var mutation in test was UB risk under cargo's parallel runner (POSIX setenv/getenv data race)	REMOVED the test rather than introduce flakes; documented why in the test module
2	Test matcher was logically broken — `bytes_out` captured but never asserted	REWROTE harness with proper Id-keyed span matching + explicit `bytes_out` assertion
3	`cap.budget_remaining=0` ambiguous (last-allowed vs rejected)	DOCUMENTED post-call semantics; integrators join `(cap.budget_remaining, error.kind)` to disambiguate
4	`process::exit(1)` killed in-flight OTel batches	ADDED `runtime.shutdown_timeout(5s)` before exit
5	`approx_value_bytes` ignored Record/Enum payloads — `bytes_out` structurally 0 for the dominant capability shape	RECURSED into `Record.fields` and `Enum.payload`

Documented limitations

• Calling `init()` twice silently overwrites the propagator (CLI is the single owner of the global subscriber)
• Empty-string `OTEL_EXPORTER_OTLP_ENDPOINT` silently disables (same behavior as unset)
• Boruna args are intentionally NOT in span attributes (privacy + size; integrators wanting this can write a custom subscriber)

Closes

• Closes [P2] Per-call observability hooks (OpenTelemetry) #9 (FleetQ P2: per-call OpenTelemetry observability)

FleetQ feedback letter status: COMPLETE

Ask	Status	PR
#3 versioned `capability_set_hash` (P1)	✅ Closed	#10
#4 streaming output from `boruna_run` (P1)	⏸ Not addressed (deferred to 0.4.0)	—
#5 structured resource limits (P1)	✅ Closed	#13
#6 stable MCP response schemas (P1)	✅ Closed	#11
#7 record/replay for net.fetch (P2)	✅ Closed	#15
#8 output schema validation gate (P2)	✅ Closed	#14
#9 OTel observability (P2)	✅ Closed	this PR
Plus 2 P0s in v0.2.0 (fine-grained policy, multi-target binaries)	✅ Shipped 2026-04-25	—

Note: #4 (streaming output) was tracked in the original feedback but not in the immediate sprint queue — defer to 0.4.0 alongside other operations work.

Next sprint pivots to `0.3-S2` (persistent state) — the critical-path 0.3.0 work that ADR PR #12 unblocks.

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