If compute_ctl is launched without a spec file, it fetches it from the
control plane with an HTTP request. We cannot get the startup tracing
context from the compute spec in that case, because we don't have it
available on start. We could still read the tracing context from the
compute spec after we have fetched it, but that would leave the fetch
itself out of the context. Pass the tracing context in environment
variables instead.
This is in preparation of using compute_ctl to launch postgres nodes
in the neon_local control plane. And seems like a good idea to
separate the public interfaces anyway.
One non-mechanical change here is that the 'metrics' field is moved
under the Mutex, instead of using atomics. We were not using atomics
for performance but for convenience here, and it seems more clear to
not use atomics in the model for the HTTP response type.
