Add tracing to vtadmin's boot function

[ajm188]

Feature Description

Right now, vtadmin takes an EnableTracing option, which enables tracing of per-{RPC,endpoint} functions. We would also like to add a root span to vtadmin's startup function. This should cover the duration of time from (1) the moment vtadmin has enough infrastructure stood up to create a span (i.e. at the very beginning of func run(...) in main.go) right up until the point where we call ListenAndServe.

Use Case(s)

More tracing! More metrics!