feat(clickhouse): implement ops.MapLength

Author: cpcloud

ibis/backends/clickhouse/compiler/values.py

@@ -1048,6 +1048,7 @@ def formatter(op, **kw):
     ops.IfNull: "ifNull",
     ops.NullIf: "nullIf",
     ops.MapContains: "mapContains",
+    ops.MapLength: "length",
     ops.MapKeys: "mapKeys",
     ops.MapValues: "mapValues",
     ops.MapMerge: "mapUpdate",