cloud: emit per-HTTP-attempt provider cost observations (method, range, bytes, status, transport error, cancelled, elapsed)

[smiggleworth]

Gap cloud-31 · missing-capability · confidence: verified · tracked in #242
Baseline: Midge 7d39f86 (0.1.1) vs Pants 2507c66. Midge paths are relative to the midge repo, Pants paths to this repo. Midge behavior is used as the proxy for the not-yet-extracted lsm-spec behavioral contract.

Prior: none (the brief's telemetry domain may also claim this)

Midge: src/storage/cloud/executor/observation.rs emits one midge::cloud_io event per attempt with method, whether it was ranged,
request/response body bytes, status, transport_error, cancelled, and elapsed_ns. It never includes URLs, keys, or credentials. Added in 4fe106c
(#286) with executor/tests/observation.rs. The same commit reduced recovery HEADs by reusing one budgeted SST reader (streaming_recovery/coverage.rs).

Pants: the provider stores emit no metrics, logs, or activities (grep for ActivitySource, Meter, ILogger, or EventSource under
src/Pants.Core/Cloud finds nothing). Runtime metrics cover WAL uploads only (Observability/Internal/RuntimeMetricsSnapshotFactory.cs:147-155).
ProviderCloudSstSourceFactory.cs:15 issues a HEAD for every SST open, and Pants has no way to measure HEAD versus ranged-GET recovery cost.

Why it matters: without per-attempt cost data, Pants cannot qualify recovery request counts, cold-read cost, or retry amplification the way Midge's evidence documents do.

Acceptance criteria:

• Each provider HTTP attempt, including retries and cancelled attempts, records method, ranged flag, request/response bytes, status, transport failure, cancellation, and elapsed time, with no URL, key, or secret.
• A test counts the HEAD and ranged-GET attempts made during a cache-loss recovery and asserts an upper bound.