Add logic to refrain from exporting un-updated metric instruments

[Brandon-Kimberly]

This PR is meant to resolve Issue #255.

I have added a new bool variable to the aggregator base class to track whether or not the aggregator has been updated in the most recent collection cycle. Some additional logic was then added to the SDK sync instrument classes to refrain from creating records from aggregators that have received no recent updates. Since there are no records created from these un-updated (stale) instruments, they will not be exported.

I did not implement the same logic for the SDK async instrument classes as I don't believe it would make sense there. Asynchronous instruments are supposed to be exported by interval rather than by request: Therefore they are never stale.

[codecov]

Codecov Report

Merging #293 into master will increase coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
+ Coverage   93.93%   94.43%   +0.50%     
==========================================
  Files         146      146              
  Lines        6564     6597      +33     
==========================================
+ Hits         6166     6230      +64     
+ Misses        398      367      -31     

Impacted Files	Coverage Δ
.../opentelemetry/sdk/metrics/aggregator/aggregator.h	58.82% <100.00%> (+2.57%)	⬆️
...emetry/sdk/metrics/aggregator/counter_aggregator.h	96.15% <100.00%> (+0.32%)	⬆️
...elemetry/sdk/metrics/aggregator/exact_aggregator.h	97.56% <100.00%> (+0.12%)	⬆️
...elemetry/sdk/metrics/aggregator/gauge_aggregator.h	100.00% <100.00%> (ø)
...etry/sdk/metrics/aggregator/histogram_aggregator.h	96.55% <100.00%> (+0.12%)	⬆️
.../metrics/aggregator/min_max_sum_count_aggregator.h	100.00% <100.00%> (ø)
...lemetry/sdk/metrics/aggregator/sketch_aggregator.h	69.76% <100.00%> (+0.71%)	⬆️
...clude/opentelemetry/sdk/metrics/sync_instruments.h	94.77% <100.00%> (+23.78%)	⬆️
sdk/test/metrics/metric_instrument_test.cc	100.00% <100.00%> (ø)

[reyang]

LGTM.