Move unbind to the Bound to simplify usage

[bogdandrutu]

I think this simplifies the usages of the bind/unbind pair.

[jkwatson]

🚀

[codecov-io]

Codecov Report

Merging #834 into master will increase coverage by 0.15%.
The diff coverage is 55.55%.

@@             Coverage Diff             @@
##             master    #834      +/-   ##
===========================================
+ Coverage     78.94%   79.1%   +0.15%     
+ Complexity      789     787       -2     
===========================================
  Files           105     105              
  Lines          2845    2838       -7     
  Branches        273     273              
===========================================
- Hits           2246    2245       -1     
+ Misses          494     489       -5     
+ Partials        105     104       -1

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/opentelemetry/metrics/DefaultMeter.java	73.68% <0%> (+2.49%)	25 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/LongMeasureSdk.java	64.7% <100%> (-1.01%)	4 <1> (-1)
...telemetry/sdk/metrics/AbstractBoundInstrument.java	54.54% <100%> (+4.54%)	3 <1> (+1)	⬆️
...io/opentelemetry/sdk/metrics/DoubleCounterSdk.java	64.7% <100%> (-1.01%)	4 <1> (-1)
...io/opentelemetry/sdk/metrics/DoubleMeasureSdk.java	64.7% <100%> (-1.01%)	4 <1> (-1)
...a/io/opentelemetry/sdk/metrics/LongCounterSdk.java	64.7% <100%> (-1.01%)	4 <1> (-1)
...elemetry/opentracingshim/SpanContextShimTable.java	96.42% <0%> (+7.14%)	8% <0%> (+1%)	⬆️

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