feat(vm): export driver traces over OTLP

[krishicks]

Summary

Continue distributed traces across the gateway-to-driver process boundary and export VM driver spans to the same OTLP/gRPC collector. The driver reports as the distinct openshell-driver-vm service.

Updated the gateway architecture and configuration reference with a generic external-driver forwarding contract.

Related Issue

Refs #2507

Changes

Instrumented:

• Every RemoteComputeDriver RPC injects the active W3C trace context into tonic metadata. Managed VM readiness and runtime initialization give startup capability probes stable parent operations rather than isolated root spans.
• A tonic service layer creates fixed, low-cardinality server spans for every ComputeDriver RPC. New handlers inherit tracing automatically; failures record OpenTelemetry error status and the gRPC status code.
• Background provisioning remains attached to CreateSandbox after the RPC returns without extending the RPC span lifetime.
• Provisioning records image preparation, bootstrap image resolution, overlay preparation, lifecycle configuration, pre-launch hooks, guest preparation, and launcher spawn as child spans.
• VM startup reconciliation roots one trace for the persisted-sandbox scan, with per-sandbox restore and provision operations beneath it. The root remains open until all spawned restore tasks finish.
• Delete cleanup records its own child operation.

Design notes:

• The gateway forwards its configured OTLP endpoint to managed external drivers. SDK OTEL_* variables continue to own sampling, batching, limits, headers, and transport tuning.
• The VM driver has its own tracer provider and service resource so trace backends preserve the service boundary.
• RPC operation names come from an explicit method mapping, keeping cardinality bounded without parsing the protobuf descriptor set at runtime.
• Propagation uses a remote SpanContext for spawned provisioning. This keeps one trace while allowing the CreateSandbox server span to finish when the RPC response is sent.
• Startup restoration is independent of gateway requests. It begins at the VM driver reconciliation span rather than attaching to an unrelated RPC.
• Existing tracing events remain on the logging path. The OpenTelemetry layer exports spans only and excludes the SDK exporter callsites to avoid recursive traces.
• Export configuration failures do not prevent the driver from serving, and buffered spans are drained during graceful shutdown.
• Trace fields identify drivers, sandboxes, images, lifecycle phases, and gRPC outcomes without recording credentials, sandbox tokens, or request query parameters.

Testing

• mise run pre-commit passes
• Unit tests added/updated
• E2E tests added/updated (if applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)
• Architecture docs updated (if applicable)

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