Update InstanaTracer to utilize kind argument

[HonakerM]

Hello,

I am working on instrumenting an application with instana and I was really confused on why the kind argument was not working for setting span kinds. I then realized it's because it's not used! This PR fixes that and automatically adds the kind if one was provided

[HonakerM]

CC: @GSVarsha @pvital @CagriYonca wasn't sure who I should ping for review/merge

[pvital]

Hello @HonakerM. Thank you for your PR.

We don't "use" the kind while starting the span, because we set this value following our internal table of types based on the span name during the closing stage of the span lifetime, when we create the Instana Span payload before transmiting it to the Agent. Check the RegisteredSpan class and you will see this setting.

The start_span() and the start_as_current_span() have the kind as argument because we are implementing the OpenTelemetry's Interface for the Tracer.

Your contribution is very useful, but still need a bit more work since it will be overwrited by the RegisteredSpan logic. In addition, changes in the unit tests will be necessary and more than welcomed.