log: avoid double span wrapping (#3833)
log: avoid double span wrapping (#3833)

Avoid wrapping a span logger inside of another span logger.

When calling `log.FromCtx`, the logger inside the context is wrapped
with a span logger. If the logger is already a span logger, it is not
wrapped.
Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion,Oncilla/scion,netsec-ethz/scion,netsec-ethz/scion,Oncilla/scion