Rename base classes to abstract class

[bogdandrutu]

Split from #831

[codecov-io]

Codecov Report

Merging #832 into master will not change coverage.
The diff coverage is 40%.

@@            Coverage Diff            @@
##             master     #832   +/-   ##
=========================================
  Coverage     78.96%   78.96%           
  Complexity      798      798           
=========================================
  Files           106      106           
  Lines          2885     2885           
  Branches        275      275           
=========================================
  Hits           2278     2278           
  Misses          503      503           
  Partials        104      104

Impacted Files	Coverage Δ	Complexity Δ
.../io/opentelemetry/sdk/metrics/LongObserverSdk.java	53.84% <ø> (ø)	2 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/LongCounterSdk.java	65.71% <ø> (ø)	5 <0> (ø)	⬇️
...o/opentelemetry/sdk/metrics/DoubleObserverSdk.java	53.84% <ø> (ø)	2 <0> (ø)	⬇️
...io/opentelemetry/sdk/metrics/DoubleCounterSdk.java	65.71% <ø> (ø)	5 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/LongMeasureSdk.java	65.71% <ø> (ø)	5 <0> (ø)	⬇️
...io/opentelemetry/sdk/metrics/DoubleMeasureSdk.java	65.71% <ø> (ø)	5 <0> (ø)	⬇️
.../opentelemetry/sdk/metrics/AbstractInstrument.java	30.43% <0%> (ø)	1 <0> (?)
...telemetry/sdk/metrics/AbstractBoundInstrument.java	50% <66.66%> (ø)	2 <1> (?)

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