Add opentracing.ref_type semantic convention.

semantic_conventions/trace/compatibility.yaml

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+groups:
+  - id: opentracing
+    prefix: opentracing
+    brief: 'This document defines semantic conventions for the OpenTracing Shim'
+    note: >
+        These conventions are used by the OpenTracing Shim layer.
+    attributes:
+      - id: ref_type
+        brief: 'Parent-child Reference type'
+        note: >
+            The causal relationship between a child Span and a parent Span.
+        type:
+          allow_custom_values: false
+          members:
+            - id: child_of
+              value: 'child_of'
+              brief: 'The parent Span depends on the child Span in some capacity'
+            - id: follows_from
+              value: 'follows_from'
+              brief: 'The parent Span does not depend in any way on the result of the child Span'

specification/trace/semantic_conventions/compatibility.md

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+# Semantic conventions for Compatibility components
+
+**Status**: [Experimental](../../document-status.md)
+
+This document defines trace semantic conventions used by the
+[compatibility](../../compatibility/) components.
+
+<!-- Re-generate TOC with `markdown-toc --no-first-h1 -i` -->
+
+<!-- toc -->
+
+- [OpenTracing](#opentracing)
+
+<!-- tocstop -->
+
+## OpenTracing
+
+`Link`s created by the OpenTracing Shim MUST set `opentracing.ref_type`
+with one of the accepted values, describing the direct causal relationships
+between a child Span and a parent Span, as defined by
+[OpenTracing](https://github.com/opentracing/specification/blob/master/specification.md).
+
+
+<!-- semconv opentracing -->
+| Attribute  | Type | Description  | Examples  | Required |
+|---|---|---|---|---|
+| `opentracing.ref_type` | string | Parent-child Reference type [1] | `child_of` | No |
+
+**[1]:** The causal relationship between a child Span and a parent Span.
+
+`opentracing.ref_type` MUST be one of the following:
+
+| Value  | Description |
+|---|---|
+| `child_of` | The parent Span depends on the child Span in some capacity |
+| `follows_from` | The parent Span does not depend in any way on the result of the child Span |
+<!-- endsemconv -->