Use LastValue by default for ValueObserver instruments

[jmacd]

The specification will state that ValueObserver uses LastValue aggregation by default. This includes adding OTLP Exporter support for LastValue aggregation.

Resolves #1139.

Part of #1177.

Corresponds with open-telemetry/opentelemetry-specification#984.

[codecov]

Codecov Report

Merging #1165 into master will increase coverage by 0.1%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #1165     +/-   ##
========================================
+ Coverage    76.4%   76.5%   +0.1%     
========================================
  Files         134     134             
  Lines        5732    5751     +19     
========================================
+ Hits         4384    4405     +21     
+ Misses       1105    1103      -2     
  Partials      243     243             

Impacted Files	Coverage Δ
exporters/otlp/internal/transform/metric.go	87.3% <ø> (+3.5%)	⬆️
sdk/metric/selector/simple/simple.go	100.0% <ø> (ø)
sdk/trace/provider.go	92.9% <0.0%> (-3.5%)	⬇️
api/trace/api.go	57.1% <0.0%> (ø)
api/global/internal/trace.go	88.8% <0.0%> (ø)
api/trace/noop_trace_provider.go	100.0% <0.0%> (ø)
semconv/http.go	95.3% <0.0%> (+<0.1%)	⬆️

[jmacd]

This corresponds with open-telemetry/opentelemetry-specification#984.