Use nullcontext; remove noop_context_manager

The comment says contextlib.nullcontext requires Python 3.6. The docs
https://docs.python.org/3/library/contextlib.html say 3.7, with
asynchronous context manager support in 3.10. But AFAICS we only use
`nullcontext` in place of a `Scope`, and that only acts as a synchronous
context manager.

synapse/logging/opentracing.py

@@ -371,13 +371,6 @@ def ensure_active_span_inner_2(*args, **kwargs):
     return ensure_active_span_inner_1
 
 
-@contextlib.contextmanager
-def noop_context_manager(*args, **kwargs):
-    """Does exactly what it says on the tin"""
-    # TODO: replace with contextlib.nullcontext once we drop support for Python 3.6
-    yield
-
-
 # Setup
 
 
@@ -472,11 +465,11 @@ def start_active_span(
     Args:
         See opentracing.tracer
     Returns:
-        scope (Scope) or noop_context_manager
+        scope (Scope) or contextlib.nullcontext
     """
 
     if opentracing is None:
-        return noop_context_manager()  # type: ignore[unreachable]
+        return contextlib.nullcontext()  # type: ignore[unreachable]
 
     if tracer is None:
         # use the global tracer by default
@@ -520,7 +513,7 @@ def start_active_span_follows_from(
         tracer: override the opentracing tracer. By default the global tracer is used.
     """
     if opentracing is None:
-        return noop_context_manager()  # type: ignore[unreachable]
+        return contextlib.nullcontext()  # type: ignore[unreachable]
 
     references = [opentracing.follows_from(context) for context in contexts]
     scope = start_active_span(
@@ -560,7 +553,7 @@ def start_active_span_from_edu(
     references = references or []
 
     if opentracing is None:
-        return noop_context_manager()  # type: ignore[unreachable]
+        return contextlib.nullcontext()  # type: ignore[unreachable]
 
     carrier = json_decoder.decode(edu_content.get("context", "{}")).get(
         "opentracing", {}

synapse/metrics/background_process_metrics.py

@@ -14,6 +14,7 @@
 
 import logging
 import threading
+from contextlib import nullcontext
 from functools import wraps
 from types import TracebackType
 from typing import (
@@ -41,11 +42,7 @@
     LoggingContext,
     PreserveLoggingContext,
 )
-from synapse.logging.opentracing import (
-    SynapseTags,
-    noop_context_manager,
-    start_active_span,
-)
+from synapse.logging.opentracing import SynapseTags, start_active_span
 from synapse.metrics._types import Collector
 
 if TYPE_CHECKING:
@@ -238,7 +235,7 @@ async def run() -> Optional[R]:
                         f"bgproc.{desc}", tags={SynapseTags.REQUEST_ID: str(context)}
                     )
                 else:
-                    ctx = noop_context_manager()
+                    ctx = nullcontext()
                 with ctx:
                     return await func(*args, **kwargs)
             except Exception: