metrics: lift restrictions on fields types

These checks stem from a rework from strv to a proper varlink object,
but the whole rework's point was to allow more expressive field types,
hence drop the unnecessary verification of the string type.

src/shared/metrics.c

@@ -161,14 +161,7 @@ static int metric_build_send(
         assert(link);
         assert(value);
 
-        if (fields) {
-                assert(sd_json_variant_is_object(fields));
-
-                _unused_ const char *k;
-                _unused_ sd_json_variant *e;
-                JSON_VARIANT_OBJECT_FOREACH(k, e, fields)
-                        assert(sd_json_variant_is_string(e));
-        }
+        assert(!fields || sd_json_variant_is_object(fields));
 
         return sd_varlink_replybo(
                         link,