Add OpenTelemetry tracing spans to VM startup pipeline

[JAORMX]

Summary

• Instrument the critical path through microvm.Run() with OpenTelemetry trace spans so consumers can identify performance bottlenecks in VM startup
• When no TracerProvider is configured (the default), all tracing is no-op with zero overhead
• Promotes go.opentelemetry.io/otel and go.opentelemetry.io/otel/trace from indirect to direct dependencies (already in the dep tree via transitive deps)

Spans added

microvm.go — root span + 8 child spans covering each sequential phase:

• microvm.Run (root) with image/name/cpus/memory attributes
• microvm.Preflight, microvm.ImagePull, microvm.RootfsClone
• microvm.RootfsHooks, microvm.BackendPrepare, microvm.NetworkStart
• microvm.VMSpawn, microvm.PostBoot

image/pull.go — sub-spans in PullWithFetcher():

• microvm.image.CacheLookup (with cache_hit attribute)
• microvm.image.Fetch, microvm.image.Extract (with layered attribute)
• microvm.image.CacheStore

ssh/client.go — span + events in WaitForReady():

• microvm.SSHWaitReady with host/port/user attributes
• ssh.probe_failed event per poll iteration with probe count

preflight/checker.go — parent + per-check spans:

• microvm.preflight.RunAll with check count
• microvm.preflight.Check per check with name/required attributes

hypervisor/libkrun/backend.go — span in Start():

• microvm.backend.Start with sub-spans for ResolveRuntime and ResolveFirmware

Motivation

Profiling brood-box startup showed "Sandbox ready" taking ~20s. The microvm.Run() call was a black box. With these spans, a consumer that configures a TracerProvider can now see exactly where time goes:

microvm.Run                                      21.451s
  microvm.Preflight                               0.000s
  microvm.ImagePull                               0.000s  (cache hit)
  microvm.RootfsClone                             5.354s  ← COW clone bottleneck
  microvm.RootfsHooks                             0.015s
  microvm.BackendPrepare                          0.000s
  microvm.NetworkStart                            0.000s
  microvm.VMSpawn                                 0.018s
  microvm.PostBoot                               16.062s
    microvm.SSHWaitReady                         16.061s  ← 8 probes @ 2s

Test plan

• go test ./... — all existing tests pass (tracing is no-op without provider)
• golangci-lint run ./... — 0 issues
• Integration: brood-box consumer with --trace flag produces expected span hierarchy

🤖 Generated with Claude Code