Change should_sample parameters to be spec compliant

[lzchen]

Fixes #1763

1. Adds SpanKind to should_sample
2. should_sample uses trace_state from the passed in Context 's SpanContext 's TraceState instead of the explicitly passed in TraceState.

[srikanthccv]

Shouldn't we also update the should_sample to take trace_state from parent_context?

[lzchen]

@lonewolf3739
I believe we already do that here

[srikanthccv]

Does it make sense to move that to should_sample and add a note saying that is the recommended way and discourage people from using trace_state param?

[lzchen]

@lonewolf3739

Does it make sense to move that to should_sample and add a note saying that is the recommended way and discourage people from using trace_state param?

This makes sense. I'll make these changes.

[lzchen]

@lonewolf3739
Please take a look.

[ocelotl]

Approving, there is mostly a minor fix in documentation suggested.