provide support for multiple processes while exporting spans

[asldevi]

implementation is based on prefixes of the span names

Reason:
we use jaeger exporter with OpenCensus php library
and there is no direct support for "SameProcessAsParentSpan" in Jaeger where as OpenCensus recommends it. Source

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[google-cla]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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