feat: configure header extraction for ASGI middleware via constructor params (#2026)

* feat: configure header extraction for ASGI middleware via constructor params

* fix django middleware

* lint

* remove import

* Fix lint

* Update instrumentation/opentelemetry-instrumentation-asgi/src/opentelemetry/instrumentation/asgi/__init__.py

util/opentelemetry-util-http/src/opentelemetry/util/http/__init__.py

@@ -12,11 +12,13 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from __future__ import annotations
+
 from os import environ
 from re import IGNORECASE as RE_IGNORECASE
 from re import compile as re_compile
 from re import search
-from typing import Iterable, List, Optional
+from typing import Callable, Iterable, Optional
 from urllib.parse import urlparse, urlunparse
 
 from opentelemetry.semconv.trace import SpanAttributes
@@ -84,9 +86,12 @@ class SanitizeValue:
         )
 
     def sanitize_header_values(
-        self, headers: dict, header_regexes: list, normalize_function: callable
-    ) -> dict:
-        values = {}
+        self,
+        headers: dict[str, str],
+        header_regexes: list[str],
+        normalize_function: Callable[[str], str],
+    ) -> dict[str, str]:
+        values: dict[str, str] = {}
 
         if header_regexes:
             header_regexes_compiled = re_compile(
@@ -216,14 +221,14 @@ def sanitize_method(method: Optional[str]) -> Optional[str]:
     return "UNKNOWN"
 
 
-def get_custom_headers(env_var: str) -> List[str]:
-    custom_headers = environ.get(env_var, [])
+def get_custom_headers(env_var: str) -> list[str]:
+    custom_headers = environ.get(env_var, None)
     if custom_headers:
-        custom_headers = [
+        return [
             custom_headers.strip()
             for custom_headers in custom_headers.split(",")
         ]
-    return custom_headers
+    return []
 
 
 def _parse_active_request_count_attrs(req_attrs):