log: avoid stack lookups when not needed/used

[ucwong]

Avoids the somewhat expensive stack.Caller invocation by checking if it is needed

[codecov]

Codecov Report

Merging #1713 (c0dcd2c) into master (eddc228) will increase coverage by 0.11%.
Report is 3 commits behind head on master.
The diff coverage is 42.30%.

❗ Current head c0dcd2c differs from pull request most recent head 1ef7bda. Consider uploading reports for the commit 1ef7bda to get more accurate results

@@            Coverage Diff             @@
##           master    #1713      +/-   ##
==========================================
+ Coverage   52.63%   52.75%   +0.11%     
==========================================
  Files         484      484              
  Lines       60637    60657      +20     
==========================================
+ Hits        31918    32000      +82     
+ Misses      26292    26224      -68     
- Partials     2427     2433       +6     

Files Changed	Coverage Δ
core/rawdb/accessors_metadata.go	0.00% <0.00%> (ø)
log/format.go	44.51% <0.00%> (-0.29%)	⬇️
common/types.go	64.58% <28.57%> (-2.84%)	⬇️
log/handler_glog.go	68.88% <100.00%> (+68.88%)	⬆️
log/logger.go	39.04% <100.00%> (+6.35%)	⬆️

... and 13 files with indirect coverage changes

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