feat: processor export phases interface

[legendecas]

Which problem is this PR solving?

• https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/sdk.md#dataflow-diagram
• To reduce penalties on delta export kind, the processors are responsible to handle delta aggregation snapshot to cumulative aggregation, and export cumulative aggregation to exporters. In this case, processors have to be notified when a new series of collect starts and finishes.

Short description of the changes

• Add start/finish method to processor interface.

[codecov]

Codecov Report

Merging #1717 (9a68d15) into master (781b30f) will decrease coverage by 0.03%.
The diff coverage is 87.17%.

@@            Coverage Diff             @@
##           master    #1717      +/-   ##
==========================================
- Coverage   91.38%   91.35%   -0.04%     
==========================================
  Files         164      165       +1     
  Lines        5059     5064       +5     
  Branches     1048     1048              
==========================================
+ Hits         4623     4626       +3     
- Misses        436      438       +2     

Impacted Files	Coverage Δ
...ages/opentelemetry-metrics/src/export/Processor.ts	100.00% <ø> (+8.00%)	⬆️
...telemetry-metrics/src/export/UngroupedProcessor.ts	83.33% <83.33%> (ø)
...ges/opentelemetry-metrics/src/export/Controller.ts	80.76% <88.88%> (+1.60%)	⬆️
packages/opentelemetry-metrics/src/Meter.ts	87.80% <100.00%> (ø)

[legendecas]

Closed due to inactivity. I will continue the investigation once there are any updates on the spec.