Add support for OTEL_PROPAGATORS

Fixes open-telemetry#1058

opentelemetry-api/setup.cfg

@@ -55,6 +55,9 @@ opentelemetry_meter_provider =
     default_meter_provider = opentelemetry.metrics:DefaultMeterProvider
 opentelemetry_tracer_provider =
     default_tracer_provider = opentelemetry.trace:DefaultTracerProvider
+opentelemetry_propagator =
+    tracecontext = opentelemetry.trace.propagation.tracecontext:TraceContextTextMapPropagator
+    baggage = opentelemetry.baggage.propagation:BaggagePropagator
 
 [options.extras_require]
 test =

opentelemetry-api/src/opentelemetry/propagators/__init__.py

@@ -53,15 +53,16 @@ def example_route():
     https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/api-propagators.md
 """
 
+from pkg_resources import iter_entry_points
+from logging import getLogger
 import typing
 
-from opentelemetry.baggage.propagation import BaggagePropagator
 from opentelemetry.context.context import Context
 from opentelemetry.propagators import composite
 from opentelemetry.trace.propagation import textmap
-from opentelemetry.trace.propagation.tracecontext import (
-    TraceContextTextMapPropagator,
-)
+from opentelemetry.configuration import Configuration
+
+logger = getLogger(__name__)
 
 
 def extract(
@@ -104,9 +105,25 @@ def inject(
     get_global_textmap().inject(set_in_carrier, carrier, context)
 
 
-_HTTP_TEXT_FORMAT = composite.CompositeHTTPPropagator(
-    [TraceContextTextMapPropagator(), BaggagePropagator()],
-)  # type: textmap.TextMapPropagator
+try:
+
+    propagators = []
+
+    for propagator in Configuration().get(
+        "PROPAGATORS", "tracecontext,baggage"
+    ).split(","):
+
+        propagators.append(
+            next(
+                iter_entry_points("opentelemetry_propagator", propagator)
+            ).load()()
+        )
+
+except Exception:  # pylint: disable=broad-except
+    logger.exception("Failed to load configured propagators")
+    raise
+
+_HTTP_TEXT_FORMAT = composite.CompositeHTTPPropagator(propagators)
 
 
 def get_global_textmap() -> textmap.TextMapPropagator: