Start an opentracing span for background processes. (#8567)

This should reduce the number of `There was no active span` errors we
see.

Fixes #8510.

changelog.d/8567.bugfix

@@ -0,0 +1 @@
+Fix increase in the number of `There was no active span...` errors logged when using OpenTracing.

synapse/metrics/background_process_metrics.py

@@ -24,6 +24,7 @@
 from twisted.internet import defer
 
 from synapse.logging.context import LoggingContext, PreserveLoggingContext
+from synapse.logging.opentracing import start_active_span
 
 if TYPE_CHECKING:
     import resource
@@ -197,14 +198,14 @@ async def run():
 
         with BackgroundProcessLoggingContext(desc) as context:
             context.request = "%s-%i" % (desc, count)
-
             try:
-                result = func(*args, **kwargs)
+                with start_active_span(desc, tags={"request_id": context.request}):
+                    result = func(*args, **kwargs)
 
-                if inspect.isawaitable(result):
-                    result = await result
+                    if inspect.isawaitable(result):
+                        result = await result
 
-                return result
+                    return result
             except Exception:
                 logger.exception(
                     "Background process '%s' threw an exception", desc,