…1176)

- Reverts #1118 (`feat(traces): [SVLS-8734] respect
Datadog-Client-Computed-Stats header`) which introduced logic to skip
backend stats when the tracer claims client-computed-stats, causing
stats to vanish entirely in some scenarios.
- Reverts #1136 (`fix(tests): remove stale _dd.compute_stats:1 assertion
from logs integration test`) which was a follow-up test adjustment for

Restores the pre-v95 behavior where `_dd.compute_stats` is set solely by
`DD_COMPUTE_TRACE_STATS_ON_EXTENSION`, keeping the backend as a safety
net.

- [x] `cargo fmt --check` passes
- [x] `cargo clippy --all-targets -- -D warnings` passes
- [x] `cargo test` — all 513 tests pass

[SVLS-8734]:
https://datadoghq.atlassian.net/browse/SVLS-8734?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

## Overview

Makes the extension respect the tracer's `Datadog-Client-Computed-Stats`
header and moves `_dd.compute_stats` from a baked-in function tag to a
per-span backend directive.

We tried supporting the `Datadog-Client-Computed-Stats` header before in
#1118, but that was reverted in #1176.

### Background

Span attribute `_dd.compute_stats` asks the backend to compute trace
stats. It must be set to `"1"` only when nobody else computed them —
neither the extension (agent) nor the tracer. Previously the extension:

1. **Baked `_dd.compute_stats` into the function tags** unconditionally
(`tags_from_env`), which also leaked the key into `_dd.tags.function`.
2. **Ignored `Datadog-Client-Computed-Stats`** entirely, so when a
tracer computed stats client-side, the backend was still asked to
compute them.

### Canonical semantics (validated against the Go agent)

The Go agent (`pkg/serverless/tags/tags.go`) only ever sets
`_dd.compute_stats = "1"`, never `"0"`, and leaves the key absent
otherwise. This PR matches that:

> Set `_dd.compute_stats="1"` iff `!compute_trace_stats_on_extension &&
!client_computed_stats`; otherwise leave it absent.

| `client_computed_stats` (header from tracer) | `compute_on_extension`
(extension config) | who computes | stamp `_dd.compute_stats="1"`?
| -------------------- | --------------------- | --------------- | ---
| true                 | (ignored)             | tracer          | ❌ no
| false                | true                  | extension       | ❌ no
| false                | false                 | backend         | ✅ yes

### Changes

- **`tags/lambda/tags.rs`** — stop baking `_dd.compute_stats` in
`tags_from_env` (no longer leaks into `_dd.tags.function`);
`COMPUTE_STATS_KEY` is now `pub` so the integration test can reuse it
instead of re-declaring the literal.
- **Path A: `traces/trace_processor.rs`** — `ChunkProcessor` gains
`client_computed_stats` and stamps `_dd.compute_stats="1"` per-span only
when neither side computes stats; the extension-side stats-generation
guard in `send_processed_traces` now also skips when
`client_computed_stats` is set.
- **Path B (extension-generated `aws.lambda` span)** —
`client_computed_stats` is propagated from the tracer's placeholder span
through `context.rs` → `processor.rs` → `processor_service.rs` →
`trace_agent.rs`, so Path B reuses the same `ChunkProcessor` stamping
(single source of truth).
- **OTLP: `otlp/agent.rs`** — the OTLP stats-generation guard previously
checked only `compute_trace_stats_on_extension`, so an OTLP request
carrying `Datadog-Client-Computed-Stats` would still generate
extension-side stats and double-count against the tracer. It now also
skips when `client_computed_stats` is set, mirroring the
`send_processed_traces` guard.
- **Single source of truth: `traces/trace_processor.rs`** — the three
decisions over the same two inputs (the per-span `_dd.compute_stats`
stamp, plus the extension-side stats-generation guards in
`send_processed_traces` and `otlp/agent.rs`) are now derived from one
`StatsComputedBy::resolve(compute_on_extension, client_computed_stats)`
helper, so the stamp and the guards can't silently drift apart.

### Note on the header value (cross-runtime)

`Datadog-Client-Computed-Stats` is not standardized (`"true"`
.NET/Java/PHP/Python, `"yes"` JS/Ruby/C++, `"t"` Go). bottlecap consumes
the already-parsed `client_computed_stats` bool from
`libdd_trace_utils`, where any non-empty value → `true`, so the fix
triggers on every runtime. A separate libdatadog PR
([DataDog/libdatadog#2071](DataDog/libdatadog#2071))
aligns the header parsing with the Go agent's `isHeaderTrue`/`ParseBool`
rules; this PR only consumes the bool and does not depend on that
change.

## Testing

- ~**Tier 0** (header-parsing contract)~ — moved to the libdatadog bump
PR #1244, since those tests assert libdatadog parsing behavior that the
`db05e1f → 48da0d8` bump changes. This branch keeps Tiers 1–3 and
rebases onto [#1244](//pull/1244) after
it merges.
- **Tier 1** (`trace_processor.rs`) — truth-table on `Span.meta`,
stats-skip guard via the real `StatsConcentratorService`, and updated
`tags.rs` unit tests asserting the key no longer appears in the tags
map. Fixed the logs/metrics integration tests that asserted the old
leak.
- **Tier 2** (`context.rs`, `processor.rs`) — context-level flag
recording and an end-to-end Path B test driving `send_ctx_spans` through
the `trace_tx` channel, asserting `_dd.compute_stats` on the
`aws.lambda` span across the truth table.
- **Tier 3** (`apm_integration_test.rs`) — full fake-intake E2E routing
a trace through `SendingTraceProcessor::send_processed_traces`: asserts
on the captured `AgentPayload` span meta and on `stats_payloads()`
(stats suppressed unless the extension computes and the tracer did not).

### ⚠️ TODO before merging

- [x] Rebase onto the libdatadog bump PR
([#1244](#1244))
after it merges.
- [ ] E2E tests.

---

> *"Computing stats twice doesn't make them twice as true, it just makes
the backend twice as grumpy."* — Claude 🤖

[APMSVLS-487](https://datadoghq.atlassian.net/browse/APMSVLS-487)

[APMSVLS-487]:
https://datadoghq.atlassian.net/browse/APMSVLS-487?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ