Require tracer to add ignored parents as links

This also adds an initial document about semantic conventions of the
link attributes.

specification/api-tracing.md

@@ -288,6 +288,14 @@ a root span, and MUST generate a new `TraceId` for each root span created.
 For a Span with a parent, the `TraceId` MUST be the same as the parent.
 Also, the child span MUST inherit all `TraceState` values of its parent by default.
 
+The option for creating a span as a root span should allow to choose whether the
+ignored parents in `Context` should be captured as links in the new span. If the
+links should be captured then the implementation SHOULD add those possible
+parents as [`Link`s](#add-links) to the new `Span`. For description of the
+ignored-parent link attributes, see [Link
+Conventions](data-link-semantic-conventions.md#ignored-parent). The capturing is
+off by default.
+
 A `Span` is said to have a _remote parent_ if it is the child of a `Span`
 created in another process. Each propagators' deserialization must set
 `IsRemote` to true on a parent `SpanContext` so `Span` creation knows if the

specification/data-link-semantic-conventions.md

@@ -0,0 +1,34 @@
+# Link Conventions
+
+This document is about standard attributes for
+[Links](api-tracing.md#add-links).
+
+## Document conventions
+
+Attributes are divided into groups, depending on the link type. Attributes in
+each group are prefixed with the link type and a dot.
+
+Certain attribute groups in this document have a **Required** column. If an
+attribute in the group that is described by the link type is marked as required
+then the attribute MUST be present in the Link. If the **Required** column is
+missing, then it is assumed that all the attributes in the group are optional.
+
+## Link types
+
+A link may be of a certain type and this can be described by the `link.type`
+attribute. The value of the attribute MUST be a string. Each type of the link
+below will provide a description with the value of the attribute.
+
+* [Ignored parent](#ignored-parent)
+
+### Ignored parent
+
+**Link type:** `ignored-parent`
+
+**Description:** These attributes are used to denote a span context that was
+ignored during a span creation as a parent and thus became only linked to the
+span.
+
+| Attribute | Description | Example | Required? |
+|---|---|---|---|
+| ignored-parent.kind | Kind of ignored span context. It MUST be either "local" or "remote". "local" denotes a span context that was a part of a current span at the time of a creation of the link. "remote" denotes a span context that was a part of remote span at the time of a creation of the link. | "local" | Yes |

specification/data-semantic-conventions.md

@@ -5,6 +5,7 @@ Span attributes.
 
 * [Resource Conventions](data-resource-semantic-conventions.md)
 * [Span Conventions](#span-conventions)
+* [Link Conventions](data-link-semantic-conventions.md)
 
 The type of the attribute SHOULD be specified in the semantic convention
 for that attribute. Array values are allowed for attributes. For