Instrument guest exec steps

[sjmiller609]

Summary

• add child spans for guest exec connection lookup, stream open, start send, and receive-until-exit steps when waiting for guest-agent readiness
• record receive-path byte counts and exit code so slow reconfigure execs can be separated from connection/setup time
• cover the detailed wait-for-agent tracing path and the no-wait path in guest exec tests

Tests

• go test ./lib/guest
• go test ./lib/instances (fails in this checkout: missing embedded hypervisor and guest-agent binaries)

---

Note

Low Risk
Observability-only changes gated on WaitForAgent; exec and retry behavior are unchanged aside from tracing hooks.

Overview
Adds OpenTelemetry child spans around each phase of a single guest exec attempt when WaitForAgent is set, so traces can separate agent-wait/retry time from connection setup vs. stream I/O vs. command runtime.

Under the existing guest.exec parent span, new steps are guest.exec.get_conn, open_stream, send_start, and recv_until_exit. The receive step records stdout/stderr byte counts, bytes sent, and exit code on success or failure. Step spans are not created when WaitForAgent is zero, avoiding duplicate noise next to API-level exec spans.

Tests use an in-memory span recorder to assert the full span tree and attributes on the wait path, and that no guest.exec* spans appear without waiting. Retry tests now clean up pooled gRPC connections via CloseConn.

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