Evaluate arguments only if they are to be logged

[jozefhajnala]

This PR proposes that arguments passed as ... are only evaluated if they are to be logged.

Motivation and background

This increases performance if logs on certain levels (e.g. TRACE) require non-negligible time to produce. One practical example is logging the hashes of files being read for reconciliation purposes. Currently, if we use:

logger::log_trace("md5=", tools::md5sum(filePath))

the md5sum will be calculated regardless of the threshold, increasing runtime even if the threshold is lower than that for TRACE and nothing will be logged.

After the change, the arguments would be evaluated only if they really are to be logged and therefore are needed.

[codecov-io]

Codecov Report

Merging #27 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   78.71%   78.61%   -0.11%     
==========================================
  Files          11       11              
  Lines         357      360       +3     
==========================================
+ Hits          281      283       +2     
- Misses         76       77       +1

Impacted Files	Coverage Δ
R/logger.R	96.11% <100%> (-0.89%)	⬇️

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[daroczig]

That's a good point, thanks!

But wouldn't it be easier to compare defintion$threshold with level early on even before defining log_arg and issue a next()?

[jozefhajnala]

Sure, would something like this be better?

[daroczig]

thanks!