Change default propagator in unconfigured API (#930)

CHANGELOG.md

@@ -9,6 +9,8 @@ release.
 
 New:
 
+- Default propagators in un-configured API must be no-op
+  ([#930](https://github.com/open-telemetry/opentelemetry-specification/pull/930)).
 - Define resource mapping for Jaeger exporters
   ([#891](https://github.com/open-telemetry/opentelemetry-specification/pull/891))
 - Add resource semantic conventions for operating systems

specification/context/api-propagators.md

@@ -267,14 +267,29 @@ Required arguments:
 
 ## Global Propagators
 
-Implementations MAY provide global `Propagator`s for
-each supported `Propagator` type.
-
-If offered, the global `Propagator`s should default to a composite `Propagator`
-containing the W3C Trace Context Propagator and the Baggage `Propagator`
-specified in [api-baggage.md](../baggage/api.md#serialization),
-in order to provide propagation even in the presence of no-op
-OpenTelemetry implementations.
+The OpenTelemetry API MUST provide a way to obtain a propagator for each
+supported `Propagator` type. Instrumentation libraries SHOULD call propagators
+to extract and inject the context on all remote calls. Propagators, depending on
+the language, MAY be set up using various dependency injection techniques or
+available as global accessors.
+
+**Note:** It is a discouraged practice, but certain instrumentation libraries
+might use proprietary context propagation protocols or be hardcoded to use a
+specific one. In such cases, instrumentation libraries MAY choose not to use the
+API-provided propagators and instead hardcode the context extraction and injection
+logic.
+
+The OpenTelemetry API MUST use no-op propagators unless explicitly configured
+otherwise. Context propagation may be used for various telemetry signals -
+traces, metrics, logging and more. Therefore, context propagation MAY be enabled
+for any of them independently. For instance, a span exporter may be left
+unconfigured, although the trace context propagation was configured to enrich logs or metrics.
+
+Platforms such as ASP.NET may pre-configure out-of-the-box
+propagators. If pre-configured, `Propagator`s SHOULD default to a composite
+`Propagator` containing the W3C Trace Context Propagator and the Baggage
+`Propagator` specified in [api-baggage.md](../baggage/api.md#serialization).
+These platforms MUST also allow pre-configured propagators to be disabled or overridden.
 
 ### Get Global Propagator