Converted TextMap propagator getter to a class and added keys method (#1196)

Co-authored-by: alrex <aboten@lightstep.com>

instrumentation/opentelemetry-instrumentation-pyramid/src/opentelemetry/instrumentation/pyramid/callbacks.py

@@ -70,7 +70,7 @@ def _before_traversal(event):
     start_time = environ.get(_ENVIRON_STARTTIME_KEY)
 
     token = context.attach(
-        propagators.extract(otel_wsgi.get_header_from_environ, environ)
+        propagators.extract(otel_wsgi.carrier_getter, environ)
     )
     tracer = trace.get_tracer(__name__, __version__)