Add initial implementation for Observers

[bogdandrutu]

No tests yet for the Observers, will add when we add the Callback support.

[codecov-io]

Codecov Report

Merging #818 into master will decrease coverage by 0.37%.
The diff coverage is 48.97%.

@@             Coverage Diff              @@
##             master     #818      +/-   ##
============================================
- Coverage     79.04%   78.66%   -0.38%     
- Complexity      785      793       +8     
============================================
  Files           104      107       +3     
  Lines          2811     2855      +44     
  Branches        269      277       +8     
============================================
+ Hits           2222     2246      +24     
- Misses          484      505      +21     
+ Partials        105      104       -1

Impacted Files	Coverage Δ	Complexity Δ
...a/io/opentelemetry/sdk/metrics/LongCounterSdk.java	58.62% <ø> (ø)	3 <0> (ø)	⬇️
...io/opentelemetry/sdk/metrics/DoubleCounterSdk.java	58.62% <ø> (ø)	3 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/LongMeasureSdk.java	58.62% <ø> (ø)	3 <0> (ø)	⬇️
...io/opentelemetry/sdk/metrics/DoubleMeasureSdk.java	58.62% <ø> (ø)	3 <0> (ø)	⬇️
...a/io/opentelemetry/sdk/metrics/BaseInstrument.java	25% <0%> (-5.77%)	1 <0> (-2)
...in/java/io/opentelemetry/sdk/metrics/MeterSdk.java	76.47% <100%> (+11.76%)	10 <2> (+2)	⬆️
...lemetry/sdk/metrics/BaseInstrumentWithBinding.java	100% <100%> (ø)	3 <3> (?)
.../io/opentelemetry/sdk/metrics/LongObserverSdk.java	42.85% <42.85%> (ø)	2 <2> (?)
...o/opentelemetry/sdk/metrics/DoubleObserverSdk.java	42.85% <42.85%> (ø)	2 <2> (?)
... and 3 more

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[jkwatson]

good next step!