opentelementry api context change hook

otel_context_hook.py

diff --git a/opentelemetry-api/src/opentelemetry/context/__init__.py b/opentelemetry-api/src/opentelemetry/context/__init__.py
index 0a2785ab..7243eb59 100644
--- a/opentelemetry-api/src/opentelemetry/context/__init__.py
+++ b/opentelemetry-api/src/opentelemetry/context/__init__.py
@@ -14,6 +14,7 @@
 
 import logging
 import typing
+from importlib import import_module
 from os import environ
 from uuid import uuid4
 
@@ -25,6 +26,38 @@ from opentelemetry.util._importlib_metadata import entry_points
 logger = logging.getLogger(__name__)
 
 
+def import_string(dotted_path: string) -> object:
+    try:
+        module_path, class_name = dotted_path.rsplit(".", 1)
+    except ValueError:
+        msg = f'"{dotted_path}" doesn\'t look like a module path'
+        raise ImportError(msg)
+
+    module = import_module(module_path)
+
+    try:
+        return getattr(module, class_name)
+    except AttributeError:
+        msg = f'Module "{module_path}" does not define a "{class_name}" attribute/class'
+        raise ImportError(msg)
+
+class DefaultHookHandler:
+    def send(self, context):
+        pass
+
+def _load_context_change_hook() -> None:
+    # FIXME: should I just use an entry point instead?
+    context_change_hook = environ.get(OTEL_PYTHON_CONTEXT_CHANGE_HOOK)
+    if context_change_hook:
+        try:
+            return import_string(context_change_hooks)()
+        except ImportError:
+            logger.exception(
+                "Failed to load context change hook",
+            )
+    return DefaultHookHandler()
+
+
 def _load_runtime_context() -> _RuntimeContext:
     """Initialize the RuntimeContext
 
@@ -67,6 +100,7 @@ def _load_runtime_context() -> _RuntimeContext:
 
 
 _RUNTIME_CONTEXT = _load_runtime_context()
+_CONTEXT_CHANGE_HOOK = _load_context_change_hook()
 
 
 def create_key(keyname: str) -> str:
@@ -140,7 +174,9 @@ def attach(context: Context) -> object:
     Returns:
         A token that can be used with `detach` to reset the context.
     """
-    return _RUNTIME_CONTEXT.attach(context)
+    token = _RUNTIME_CONTEXT.attach(context)
+    _CONTEXT_CHANGE_HOOK.send(context)
+    return token
 
 
 def detach(token: object) -> None:
@@ -154,6 +190,8 @@ def detach(token: object) -> None:
         _RUNTIME_CONTEXT.detach(token)
     except Exception:  # pylint: disable=broad-except
         logger.exception("Failed to detach context")
+    else:
+        _CONTEXT_CHANGE_HOOK.send(get_current())
 
 
 # FIXME This is a temporary location for the suppress instrumentation key.