Apply breaking changes to Span API (from Sampling OTEP 0006)

[Oberon00]

Even though we don't support sampling yet, the following already applies:

3. Add capability to record Attributes that can be used for sampling decision during the Span
   creation time.
4. Remove addLink APIs from the Span interface, and allow recording links only during the span
   construction time.

Reference:

• Approval PR: Change status to approved for 0006-sampling RFC. oteps#28
• OTEP text: https://github.com/open-telemetry/oteps/blob/master/text/0006-sampling.md

[Oberon00]

Related #134 (also part of the same OTEP, but that one is not breaking)

[codeboten]

When removing add_link from the api, i'm assuming we'll also want to remove add_lazy_link as well:

opentelemetry-python/opentelemetry-api/src/opentelemetry/trace/__init__.py

Lines 201 to 216 in e53edd5

	def add_link(
	self,
	link_target_context: "SpanContext",
	attributes: types.Attributes = None,
	) -> None:
	"""Adds a `Link` to another span.
	Adds a single `Link` from this Span to another Span identified by the
	`SpanContext` passed as argument.
	"""
	def add_lazy_link(self, link: "Link") -> None:
	"""Adds a `Link` to another span.
	Adds a `Link` that has previously been created.
	"""

[codeboten]

Started the work here #259

[codeboten]

On the following:

3. Add capability to record Attributes that can be used for sampling decision during the Span
   creation time.

It's currently possible for a Sampler to pass attributes when creating a Decision. This sampling decision's attributes are then passed into a Span. This approach seems to address the otep. Any thoughts?

[Oberon00]

@codeboten No, the OTEP is confusingly worded here: This is about adding span attributes right when creating the span so that the sampler can e.g. sample out all spans that have a http.url that starts with http://example.com/healthcheck. Normally you would call start_span first and the set_attribute on the span. The OTEP basically requires start_span to have an additional parameter to accept span attributes right away, and for these attributes to be also provided to the sampler.

[Oberon00]

This OTEP was also already "implemented" in the spec, see https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-tracing.md#span-creation:

The API MUST accept the following parameters:
[...]

• Attributes - similar API with Span::SetAttributes. These attributes will be used to make a sampling decision as noted in sampling description. Empty list will be assumed if not specified.