Emit machine_id for producers on remote hosts

This revision adds machine_id (a monotonic counter) to trace packets
to identify trace data from remote hosts (e.g. VM guests or remote
physical machines):

1. The relay service sets machine_id_hint in the SetPeerIdentity message
that can be used to identity different machines.
2. HostImpl generates base::MachineID from machine_id_hint if the relay
service sets it, or fallback to socket hostname.
3. MachineID is passed over in the tracing core.
4. In the core service, machine IDs are emitted to trace packets.

Bug: 284258446

Change-Id: Ie8287c026976938b90adc72df97538246d48a100