Add Context arg to SpanProcessor.OnStart.

[Oberon00]

Fixes #1785.

[Oberon00]

This seems a bit dirty, so if someone can suggest a better solution that would be good.

[jkwatson]

I think it's fine for now, although agree it's ugly. I would like to revisit the way the disruptor messages are built at some point, since I ran into difficulties when trying to work with it a few months ago, as well. I think the best answer might be to create a Tuple to hold the message data, or maybe split the message types so they can be more easily customized.

[codecov]

Codecov Report

Merging #1792 into master will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@             Coverage Diff              @@
##             master    #1792      +/-   ##
============================================
+ Coverage     85.94%   85.96%   +0.02%     
  Complexity     1379     1379              
============================================
  Files           166      166              
  Lines          5307     5308       +1     
  Branches        549      549              
============================================
+ Hits           4561     4563       +2     
+ Misses          551      550       -1     
  Partials        195      195              

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/opentelemetry/sdk/trace/SpanBuilderSdk.java	96.80% <ø> (ø)	31.00 <0.00> (ø)
...telemetry/sdk/trace/export/BatchSpanProcessor.java	89.18% <0.00%> (ø)	8.00 <0.00> (ø)
...elemetry/sdk/trace/export/SimpleSpanProcessor.java	92.30% <ø> (ø)	12.00 <0.00> (ø)
...io/opentelemetry/sdk/trace/MultiSpanProcessor.java	94.44% <100.00%> (ø)	17.00 <0.00> (ø)
.../io/opentelemetry/sdk/trace/NoopSpanProcessor.java	100.00% <100.00%> (ø)	8.00 <1.00> (ø)
...ntelemetry/sdk/trace/RecordEventsReadableSpan.java	81.04% <100.00%> (ø)	67.00 <0.00> (ø)
...ions/trace/export/DisruptorAsyncSpanProcessor.java	85.41% <100.00%> (ø)	10.00 <0.00> (ø)
...k/extensions/trace/export/DisruptorEventQueue.java	81.52% <100.00%> (+0.20%)	13.00 <0.00> (ø)
...k/extensions/trace/jaeger/sampler/RateLimiter.java	94.11% <0.00%> (-5.89%)	4.00% <0.00%> (-1.00%)
...elemetry/opentracingshim/SpanContextShimTable.java	96.42% <0.00%> (+7.14%)	8.00% <0.00%> (+1.00%)

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[jkwatson]

Thanks!

[anuraaga]

Thanks!