The log.Debug function takes a ...interface{} argument which must be
heap allocated. This is done even if debug logging is disabled, which
is the common case. Add a log.DebugEnabled() function to check if it
is enabled, and skip allocating this slice in the common case. The
benchstat output from my laptop shows this saves about 5% CPU and
2 allocs/call for the BenchmarkStartSpan microbenchmark.

Normally, I do not like this sort of ugly log level check. However,
given that StartSpan is widely called, and sometimes from hot code
paths, this may be worthit.

name         old time/op    new time/op    delta
StartSpan-8    1.63µs ± 5%    1.55µs ±10%   -5.00%  (p=0.003 n=9+9)

name         old alloc/op   new alloc/op   delta
StartSpan-8    1.55kB ± 0%    1.52kB ± 0%   -2.08%  (p=0.000 n=10+8)

name         old allocs/op  new allocs/op  delta
StartSpan-8      19.0 ± 0%      17.0 ± 0%  -10.53%  (p=0.000 n=10+10)