Allow spam-checker modules to be provide async methods. (#8890)

Spam checker modules can now provide async methods. This is implemented
in a backwards-compatible manner.

synapse/metrics/background_process_metrics.py

@@ -13,7 +13,6 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import inspect
 import logging
 import threading
 from functools import wraps
@@ -25,6 +24,7 @@ from twisted.internet import defer
 
 from synapse.logging.context import LoggingContext, PreserveLoggingContext
 from synapse.logging.opentracing import noop_context_manager, start_active_span
+from synapse.util.async_helpers import maybe_awaitable
 
 if TYPE_CHECKING:
     import resource
@@ -206,12 +206,7 @@ def run_as_background_process(desc: str, func, *args, bg_start_span=True, **kwar
                 if bg_start_span:
                     ctx = start_active_span(desc, tags={"request_id": context.request})
                 with ctx:
-                    result = func(*args, **kwargs)
-
-                    if inspect.isawaitable(result):
-                        result = await result
-
-                    return result
+                    return await maybe_awaitable(func(*args, **kwargs))
             except Exception:
                 logger.exception(
                     "Background process '%s' threw an exception", desc,