Allow setting span start/end times explicitly

We have a use case where the start/end times of traces is coming from an external system. We need to be able to explicitly set the times instead of the times automatically retrieved from the local clock.

The suggested change to the Span API is the following:

1. Add void setStartTime(Timestamp startTimestamp) to Span.
2. Add void end(EndSpanOptions options, Timestamp endTimestamp) to Span.

Here is the draft of proposed solution that I can turn into Pull Request if this issue is accepted:

https://github.com/census-instrumentation/opencensus-java/compare/master...tigrannajaryan:feature/tigran/span-times?expand=1

[tigrannajaryan]

I am constructing Spans, I didn’t consider creating SpanData directly. I can check that to see if it works for my use case.

For reference here is how I use this feature in my code currently:

Span span = tracer.spanBuilderWithExplicitParent(operationName, parentSpan).startSpan();
span.setStartTime(Timestamp.fromMicros(startTimeMicros));
span.end(EndSpanOptions.DEFAULT, Timestamp.fromMicros(endTimeMicros));

[tigrannajaryan]

@MahatmaFatalError sorry, I no longer have that piece of code around to dig into it. I solved my problem in a different (unrelated) way, so can’t help much with this.