feat: force flush and shutdown callback for span exporters

[dyladan]

Fixes #1295 and is related to #1290

[codecov]

Codecov Report

Merging #1296 into master will decrease coverage by 0.02%.
The diff coverage is 71.87%.

@@            Coverage Diff             @@
##           master    #1296      +/-   ##
==========================================
- Coverage   93.24%   93.21%   -0.03%     
==========================================
  Files         137      133       -4     
  Lines        3878     3891      +13     
  Branches      790      801      +11     
==========================================
+ Hits         3616     3627      +11     
- Misses        262      264       +2     

Impacted Files	Coverage Δ
...ges/opentelemetry-tracing/src/NoopSpanProcessor.ts	42.85% <0.00%> (-17.15%)	⬇️
...elemetry-tracing/src/export/SimpleSpanProcessor.ts	82.35% <80.00%> (+3.78%)	⬆️
...telemetry-tracing/src/export/BatchSpanProcessor.ts	92.15% <81.81%> (-3.59%)	⬇️
...es/opentelemetry-tracing/src/MultiSpanProcessor.ts	90.90% <83.33%> (-9.10%)	⬇️
...elemetry-semantic-conventions/src/trace/general.ts
...lemetry-semantic-conventions/src/trace/database.ts
...entelemetry-semantic-conventions/src/trace/http.ts
...pentelemetry-semantic-conventions/src/trace/rpc.ts

[obecny]

lgtm, just please fix the import vs require for sinon

[obecny]

Just looking at coverage it would be good to add more unit tests