add setOpenTracingParentSpanContext to OpenTracingBackend

[arun0009]

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Adding setOpenTracingParentSpanContext to OpenTracingBackend as we don't usually have the current span available and have to build span context from B3 headers (e.g. in the case of Zipkin).

If we create a new Span just so we can pass it to OpenTracingBackend.setOpenTracingParentSpan we have unnecessary additional trace/noise in tracing output.

Also, there's no way to get Span out from SpanContext per this issue opentracing/specification#81.

[adamw]

Released in 3.0.0-RC15, thanks!