Add an infrastructure for usage logging.

[ueshin]

No description provided.

[codecov-io]

Codecov Report

Merging #494 into master will increase coverage by 0.02%.
The diff coverage is 93.19%.

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
+ Coverage   93.71%   93.74%   +0.02%     
==========================================
  Files          28       30       +2     
  Lines        3803     3948     +145     
==========================================
+ Hits         3564     3701     +137     
- Misses        239      247       +8

Impacted Files	Coverage Δ
databricks/koalas/usage_logging/usage_logger.py	100% <100%> (ø)
databricks/conftest.py	96.96% <100%> (+0.54%)	⬆️
databricks/koalas/namespace.py	89.91% <100%> (+0.04%)	⬆️
databricks/koalas/__init__.py	84.21% <63.63%> (-8.9%)	⬇️
databricks/koalas/usage_logging/__init__.py	94.33% <94.33%> (ø)
databricks/koalas/datetimes.py	96.55% <0%> (+1.72%)	⬆️

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[softagram-bot]

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