Refactor exporter - step 10

[reyang]

This PR concludes #1078.

Changes

• Updated the processor/exporter to use sync version of Shutdown and ForceFlush.
• Added timeout to all the shutdown/flush methods.

For significant contributions please make sure you have completed the following items:

• Design discussion issue #
• Changes in public API reviewed

[codecov]

Codecov Report

Merging #1135 into master will decrease coverage by 0.04%.
The diff coverage is 41.17%.

@@            Coverage Diff             @@
##           master    #1135      +/-   ##
==========================================
- Coverage   77.28%   77.23%   -0.05%     
==========================================
  Files         220      220              
  Lines        6260     6269       +9     
==========================================
+ Hits         4838     4842       +4     
- Misses       1422     1427       +5     

Impacted Files	Coverage Δ
...rc/OpenTelemetry.Exporter.Jaeger/JaegerExporter.cs	69.76% <ø> (ø)
src/OpenTelemetry/Trace/ActivityExporter.cs	80.00% <ø> (ø)
...penTelemetry/Trace/BatchExportActivityProcessor.cs	37.20% <0.00%> (+0.84%)	⬆️
...elemetry/Trace/ReentrantExportActivityProcessor.cs	28.57% <ø> (+1.29%)	⬆️
src/OpenTelemetry/Trace/ActivityProcessor.cs	64.70% <50.00%> (+6.81%)	⬆️
.../OpenTelemetry/Trace/CompositeActivityProcessor.cs	79.72% <56.52%> (-11.94%)	⬇️
...us/Implementation/PrometheusExporterEventSource.cs	63.63% <0.00%> (-9.10%)	⬇️
...ZPages/Implementation/ZPagesExporterEventSource.cs	62.50% <0.00%> (+6.25%)	⬆️

[CodeBlanch]

nit: Should we spell it out fully timeoutMilliseconds?

[reyang]

That name is coming from the spec.

@cijothomas if we want be C# idiomatic, I will use timeoutMilliseconds across the entire repo (current many places are already using the timeoutMillis convention from the spec.

[reyang]

I've changed all places to use timeoutMilliseconds.

[CodeBlanch]

LGTM, just the one nit on timeoutMillis name.