Add setting to disable shared spans (joins) in tracer.

[tylerbenson]

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Other tracer implementations include a setting to force client and server spans to have different spanId's. In Go the setting is WithSharedSpans(false) and in Java it's supportsJoin(false).

There should be something similar in ruby.

This is important because zipkin traces may be reported to non-zipkin backends that might not support the concept of joining spans.

[dpdpbj]

agree. I encountered a problem when use jaeger client send request to zipkin client, and the backend server is jaeger server. It will report the duplicate span ID.

[jcchavezs]

I think the main reason here is feature parity with other languages. Whether Jaeger supports it or not might be discussed in their forums.

@dp9463 are you up for a PR?