Make instrument's builder private class

[bogdandrutu]

No description provided.

[codecov-io]

Codecov Report

Merging #819 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #819      +/-   ##
============================================
+ Coverage     79.04%   79.11%   +0.07%     
- Complexity      785      790       +5     
============================================
  Files           104      104              
  Lines          2811     2811              
  Branches        269      269              
============================================
+ Hits           2222     2224       +2     
+ Misses          484      483       -1     
+ Partials        105      104       -1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/opentelemetry/sdk/metrics/MeterSdk.java	64.7% <100%> (ø)	8 <4> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/LongCounterSdk.java	58.62% <100%> (ø)	4 <1> (+1)	⬆️
...io/opentelemetry/sdk/metrics/DoubleCounterSdk.java	58.62% <100%> (ø)	4 <1> (+1)	⬆️
...a/io/opentelemetry/sdk/metrics/LongMeasureSdk.java	58.62% <100%> (ø)	4 <1> (+1)	⬆️
...io/opentelemetry/sdk/metrics/DoubleMeasureSdk.java	58.62% <100%> (ø)	4 <1> (+1)	⬆️
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	91.51% <0%> (+1.21%)	45% <0%> (+1%)	⬆️

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[bogdandrutu]

Trivial change, merging