Setting requests in progress multiprocess mode as livesum

By default it was by pid, this could generate a lot of metrics in case
of many workers and multiprocess.

starlette_prometheus/middleware.py

@@ -8,7 +8,6 @@
 from starlette.routing import Match
 from starlette.status import HTTP_500_INTERNAL_SERVER_ERROR
 from starlette.types import ASGIApp
-
 REQUESTS = Counter(
     "starlette_requests_total", "Total count of requests by method and path.", ["method", "path_template"]
 )
@@ -31,6 +30,7 @@
     "starlette_requests_in_progress",
     "Gauge of requests by method and path currently being processed",
     ["method", "path_template"],
+    multiprocess_mode='livesum'
 )