Remove labels keys from instruments

[bogdandrutu]

See open-telemetry/opentelemetry-specification#565

[codecov-io]

Codecov Report

Merging #1121 into master will decrease coverage by 0.13%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master    #1121      +/-   ##
============================================
- Coverage     85.68%   85.54%   -0.14%     
+ Complexity     1081     1078       -3     
============================================
  Files           138      138              
  Lines          3974     3965       -9     
  Branches        351      351              
============================================
- Hits           3405     3392      -13     
- Misses          428      431       +3     
- Partials        141      142       +1     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/opentelemetry/metrics/DefaultMeter.java	70.29% <ø> (-0.58%)	9.00 <0.00> (ø)
...etry/contrib/metrics/runtime/GarbageCollector.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
...telemetry/contrib/metrics/runtime/MemoryPools.java	0.00% <ø> (ø)	0.00 <0.00> (ø)
.../opentelemetry/sdk/metrics/AbstractInstrument.java	94.23% <100.00%> (-0.42%)	10.00 <0.00> (ø)
...pentelemetry/sdk/metrics/InstrumentDescriptor.java	100.00% <100.00%> (ø)	2.00 <1.00> (ø)
...src/main/java/io/opentelemetry/internal/Utils.java	78.18% <0.00%> (-9.10%)	24.00% <0.00%> (-3.00%)
...elemetry/sdk/trace/export/BatchSpansProcessor.java	91.72% <0.00%> (-1.51%)	9.00% <0.00%> (ø%)

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