Hi in metabase is there a way we could define custom schemas like dimensions, calculated metrics from the connected db just like LookML structure so we can reuse them.
I’m going to ping @sbelak because this is his wheelhouse 
I know MBTL was implemented a couple of months ago, but not sure if that’s what you’re after.
From what I know, you need to create a .yaml file and place it in ./resources/domain_entity_specs/, but I’m not entirely sure about the structure besides that.
Issues about similar thing, but different areas:
https://github.com/metabase/metabase/issues/7459
https://github.com/metabase/metabase/issues/9434
We’re moving into the direction of allowing you to define transformations/views for your data as well as have a better semantic layer over the actual data (eg. abstract over various topologies of transactions-like data and treat them all the same by automagically & seamlessly generating scaffolding transformations), but none of that is exposed yet, let alone production ready.
For simple calculated metrics and filters you can try metrics and segments in admin->data model.