Define Propagation-only Span to simplify active Span logic in Context. (

#994)

* Define DefaultSpan to simplify active Span in Context.

* Update CHANGELOG.

* Update specification/trace/api.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

* Update specification/trace/api.md

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

* Apply feedback.

* Specify the SDK must not override it.

* Update CHANGELOG.

* Rename to PropagatedSpan.

* More feedback.

Co-authored-by: Christian Neumüller <christian+github@neumueller.me>

CHANGELOG.md

@@ -31,6 +31,8 @@ New:
     [#946](https://github.com/open-telemetry/opentelemetry-specification/pull/946))
 - Update the header name for otel baggage, and version date
   ([#981](https://github.com/open-telemetry/opentelemetry-specification/pull/981))
+- Define PropagationOnly Span to simplify active Span logic in Context
+  ([#994](https://github.com/open-telemetry/opentelemetry-specification/pull/994))
 
 Updates:
 

specification/trace/api.md

@@ -32,6 +32,7 @@ Table of Contents
     * [End](#end)
     * [Record Exception](#record-exception)
   * [Span lifetime](#span-lifetime)
+  * [Propagated Span creation](#propagated-span-creation)
 * [Status](#status)
   * [StatusCanonicalCode](#statuscanonicalcode)
   * [Status creation](#status-creation)
@@ -363,18 +364,13 @@ parent is remote.
 
 #### Determining the Parent Span from a Context
 
-When a new `Span` is created from a `Context`, the `Context` may contain:
+When a new `Span` is created from a `Context`, the `Context` may contain a `Span`
+representing the currently active instance, and will be used as parent.
+If there is no `Span` in the `Context`, the newly created `Span` will be a root span.
 
-- A current `Span`
-- An extracted `SpanContext`
-- A current `Span` and an extracted `SpanContext`
-- Neither a current `Span` nor an extracted `Span` context
-
-The parent should be selected in the following order of precedence:
-
-- Use the current `Span`, if available.
-- Use the extracted `SpanContext`, if available.
-- There is no parent. Create a root `Span`.
+A `SpanContext` cannot be set as active in a `Context` directly, but through the use
+of a [Propagated Span](#propagated-span-creation) wrapping it.
+For example, a `Propagator` performing context extraction may need this.
 
 #### Add Links
 
@@ -577,6 +573,24 @@ timestamps to the Span object:
 Start and end time as well as Event's timestamps MUST be recorded at a time of a
 calling of corresponding API.
 
+### Propagated Span creation
+
+The API MUST provide an operation for wrapping a `SpanContext` with an object
+implementing the `Span` interface. This is done in order to expose a `SpanContext`
+as a `Span` in operations such as in-process `Span` propagation.
+
+If a new type is required for supporting this operation, it SHOULD be named `PropagatedSpan`.
+
+The behavior is defined as follows:
+
+- `GetContext()` MUST return the wrapped `SpanContext`.
+- `IsRecording` MUST return `false` to signal that events, attributes and other elements
+  are not being recorded, i.e. they are being dropped.
+
+The remaining functionality of `Span` MUST be defined as no-op operations.
+
+This functionality MUST be fully implemented in the API, and SHOULD NOT be overridable.
+
 ## Status
 
 `Status` interface represents the status of a finished `Span`. It's composed of