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

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

instrumentation/opentelemetry-instrumentation-flask/src/opentelemetry/instrumentation/flask/__init__.py

@@ -118,7 +118,7 @@ def _before_request():
     if span_name is None:
         span_name = otel_wsgi.get_default_span_name(environ)
     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__)