Lazy logging macros that only evaluate the message expression if the log level is enabled.
This avoids the cost of format!(...) when the level is disabled.
Cheap, cached effective max log verbosity (0=Off,1=Error,2=Warn,3=Info,4=Debug,5=Trace).
Read on every lazy-log macro invocation to short-circuit the (expensive, RwLock-guarded)
reload-subscriber enabled check when the level is disabled — the common hot-path case
(e.g. per-command trace/debug logs while running at the default Warn level). Updated by
init whenever the effective level changes; set as an UPPER BOUND on the true effective
level so it can never suppress a log that should be emitted.
Attempt to read a directory path from an environment variable. If the environment variable envname exists
and contains a valid path - this function will create and return that path. In any case of failure,
this method returns None (e.g. the environment variable exists but contains an empty path etc)