feat: allow flush level OFF

[daniel-sanche]

The library takes in a FlushLevel argument, which defaults to ERROR. Any logs at that level or higher will be auto-flushed instead of batched.

It appears there is no way to turn the flush level off, forcing all logs to be batched. This PR checks for Level.OFF, and doesn't set the flush level if it is chosen

Fixes #133 ☕️

[codecov]

Codecov Report

Merging #319 (3f91889) into master (c0e02ba) will increase coverage by 0.39%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #319      +/-   ##
============================================
+ Coverage     69.03%   69.42%   +0.39%     
- Complexity       40       41       +1     
============================================
  Files             3        3              
  Lines           155      157       +2     
  Branches         17       18       +1     
============================================
+ Hits            107      109       +2     
  Misses           33       33              
  Partials         15       15              

Impacted Files	Coverage Δ	Complexity Δ
.../google/cloud/logging/logback/LoggingAppender.java	67.85% <100.00%> (+0.46%)	32.00 <0.00> (+1.00)

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