add random sample

add MultiIOWriter

add Header callback to FileWriter (phuslu#42)

add example to add context fields. Closes phuslu#31 (phuslu#40)

Added example to add context fields without changing original log configuration

time.now performance is improved, see golang/go#45819

tweak logger test

sync latest bencmark results from github actions

bugfix for memory corruption of formatter

bugfix for interface

algin TimeDiff to Dur