fix(metrics): bound observers' value not been updated

[legendecas]

Which problem is this PR solving?

• bound observer's value not been updated, i.e. observer function not been called.

Short description of the changes

• bound observers' observer result should be inherited from its original observer and been updated along with ObserverMetric.

[codecov-io]

Codecov Report

Merging #994 into master will increase coverage by 0.03%.
The diff coverage is 96.96%.

@@            Coverage Diff             @@
##           master     #994      +/-   ##
==========================================
+ Coverage   94.90%   94.94%   +0.03%     
==========================================
  Files         212      212              
  Lines        8676     8701      +25     
  Branches      791      790       -1     
==========================================
+ Hits         8234     8261      +27     
+ Misses        442      440       -2     

Impacted Files	Coverage Δ
packages/opentelemetry-metrics/src/Metric.ts	96.87% <ø> (ø)
...ckages/opentelemetry-metrics/src/ObserverResult.ts	92.85% <85.71%> (-7.15%)	⬇️
...kages/opentelemetry-metrics/src/BoundInstrument.ts	100.00% <100.00%> (+7.14%)	⬆️
packages/opentelemetry-metrics/test/Meter.test.ts	99.39% <100.00%> (+0.03%)	⬆️

[dyladan]

This is not a bug as there should in fact be no concept of a "Bound Observer" according to the spec
https://github.com/open-telemetry/oteps/blob/master/text/0072-metric-observer.md#observer-calling-conventions

There is no equivalent of a "bound" observer instrument as there is with Counter, Gauge, and Measure instruments.

Closing this and opening an issue to remove the bound observer.