fix: Fix pg_stat_archiver on pg13- (#235)

On older postgres version the existing cast to numeric isn't enough to
guarantee that the result expression is numeric too, so explicitly cast the
resulting expression to numeric to make sure that we can round the result.

powa/server.py

@@ -722,10 +722,11 @@ class GlobalArchiverMetricGroup(MetricGroupDef):
     @property
     def query(self):
         query = powa_get_archiver_sample()
+        ts = get_ts()
 
         cols = [
             "extract(epoch FROM ts) AS ts",
-            "round(nb_arch::numeric / " + get_ts() + ", 2) AS nb_arch",
+            "round((nb_arch::numeric / " + ts + ")::numeric, 2) AS nb_arch",
             "nb_to_arch",
         ]