Reduce tracing volume for read-heavy paths

[sjmiller609]

Summary

• sample successful root GET traces at 10% by default via OTEL config
• keep sampled-out GET errors visible with a compact fallback HTTP span
• suppress noisy vm info tracing on read-heavy status paths while keeping outer GetVMInfo error visibility

Testing

• go test ./lib/otel ./lib/hypervisor ./lib/vmm ./cmd/api/config
• go test ./lib/hypervisor/firecracker -run TestNonexistent -count=0
• GOOS=darwin GOARCH=arm64 go test ./lib/hypervisor/vz -run TestNonexistent -count=0
• go test ./cmd/api -run TestNonexistent -count=0

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Note

Medium Risk
Changes OpenTelemetry sampling and selectively suppresses hypervisor/VMM HTTP spans, which could hide useful debugging data if misconfigured, but does not affect request handling or persistence paths.

Overview
Reduces OpenTelemetry trace volume by adding configurable sampling for successful server GET requests via otel.successful_get_sample_ratio (default 0.1), wiring it through config/env, validation, initialization, and documentation.

Suppresses tracing for noisy hypervisor status GET calls (notably GET / and GET /api/v1/vm.info) across Firecracker, VZ shim, and Cloud Hypervisor VMM HTTP clients, while adding an error-only detached span for GetVMInfo failures to preserve visibility. Includes targeted unit tests to lock in the new defaults and suppression/sampling behavior.

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