fix: performance metrics queries

[vasco-santos]

Improves metrics related queries to get the API endpoint working:

• supabase count was not properly doing the count, but getting all the rows instead, which was too slow.
• added an unique index to cid including dag_size to speed up pin_dag_size_total rpc function.

All thanks to @ribasushi

New function already created into production DB

[ribasushi]

@vasco-santos perhaps the problem was that the '*' was supposed to be 'count(*)' who knows...

[vasco-santos]

@ribasushi per the docs no https://supabase.io/docs/reference/javascript/select#querying-with-count-option , but yeah, let's just use the SQL function