[*] refactor reco_add_index metric (#1117)

* Refactor `reco_add_index` metric

split the result into separate rows in the
form `query_id | recommended_index_ddl ...`.

order queries by total_exec_time from
pg_stat_statements.

* Add new `Missing Indexes` panel to `Index overview` dashboard

* Return only 1 row per distinct recommendation in `reco_add_index`

* Align column names and sizes in `Invalid indexes` panel with the rest.

* Increase `min_selectivity` for index advisor

* Update `Missing Indexes` panel description

* Use `min_selectivty => 90` in `reco_add_index`

* Improve `Missing Indexes` panel query.

Its enough to rely on `max(time)` to get the latest
fetch results.

* Improve 'Missing Indexes' panel description

Author: 0xgouda