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

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

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

@@ -54,6 +54,7 @@ from opentelemetry.instrumentation.utils import (
     http_status_to_status_code,
     unwrap,
 )
+from opentelemetry.trace.propagation.textmap import DictGetter
 from opentelemetry.trace.status import Status
 from opentelemetry.util import ExcludeList, time_ns
 
@@ -84,6 +85,8 @@ def get_traced_request_attrs():
 _excluded_urls = get_excluded_urls()
 _traced_attrs = get_traced_request_attrs()
 
+carrier_getter = DictGetter()
+
 
 class TornadoInstrumentor(BaseInstrumentor):
     patched_handlers = []
@@ -185,13 +188,6 @@ def _log_exception(tracer, func, handler, args, kwargs):
     return func(*args, **kwargs)
 
 
-def _get_header_from_request_headers(
-    headers: dict, header_name: str
-) -> typing.List[str]:
-    header = headers.get(header_name)
-    return [header] if header else []
-
-
 def _get_attributes_from_request(request):
     attrs = {
         "component": "tornado",
@@ -218,9 +214,7 @@ def _get_operation_name(handler, request):
 
 def _start_span(tracer, handler, start_time) -> _TraceContext:
     token = context.attach(
-        propagators.extract(
-            _get_header_from_request_headers, handler.request.headers,
-        )
+        propagators.extract(carrier_getter, handler.request.headers,)
     )
 
     span = tracer.start_span(