no-op StartSpan: always return the parent

[tsloughter]

Changes

This change makes it so no span id generation is required in the API's no-op tracer. My understanding from the discussion back when this part was added to the spec the implementation is supposed to create a new span id in the no-op tracer if the parent is a recording span -- while saying this is almost never the case it still would require it to be possible.

This is what we had implemented in the Erlang API:

    %% Spec: Behavior of the API in the absence of an installed SDK
    case otel_tracer:current_span_ctx(Ctx) of
        Parent=#span_ctx{trace_id=TraceId,
                         is_recording=false} when TraceId =/= 0 ->
            %% if the parent is a non-recording Span it MAY return the parent Span
            Parent;
        Parent=#span_ctx{trace_id=TraceId} when TraceId =/= 0 ->
            %% API MUST create a non-recording Span with the SpanContext in the parent Context
            SpanCtx = Parent#span_ctx{span_id=opentelemetry:generate_span_id(),
                                      is_recording=false},
            SpanCtx;
        _ ->
            %% If the parent Context contains no valid Span,
            %% an empty non-recording Span MUST be returned
            ?NOOP_SPAN_CTX
    end.

After implementing configurable IdGenerators which required moving the id generators to the SDK, https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/trace/sdk.md#id-generators, I realized the above code could no longer be possible and would not make sense even if it were.

Go and Java both always return the parent or the empty non-recording span, and maybe others. If this is the case for those two and others then this change is an attempt to bring the spec inline with existing implementations. So I think that makes it a non-breaking change :)

[github-actions]

This PR was marked stale due to lack of activity. It will be closed in 7 days.

[carlosalberto]

Hey, we forgot to add a CHANGELOG entry! @tsloughter think you could cook one?

[tsloughter]

@carlosalberto woops, yup, will open a PR in a minute.