Update README with better user grants guidance (#4528)

Update README with better user grants guidance

Added notes on grants for the `db-o11y` to access specific schemas.

Author: cristiangreco

internal/component/database_observability/README.md

@@ -400,21 +400,44 @@ show track_activity_query_size;
  4kB
 ```
 
-6. Create a dedicated DB user and grant permissions.
+6. Create a dedicated DB user and grant permissions to monitor the DB.
 
 ```sql
 CREATE USER "db-o11y" WITH PASSWORD '<password>';
 GRANT pg_monitor TO "db-o11y";
 GRANT pg_read_all_stats TO "db-o11y";
 ```
 
-7. Verify that the user has been properly created.
+7. Verify that the user has been properly created and has the correct privileges for the `pg_stat_statements` extension.
 
 ```sql
 -- run with the `db-o11y` user
 SELECT * FROM pg_stat_statements LIMIT 1;
 ```
 
+8. Grant the `db-o11y` user additional privileges to access the objects (databases, schemas, tables, views) for which you want to collect detailed information.
+
+For example, connect to a `payments` database and grant access to specific schemas:
+
+```sql
+-- switch to the 'payments' database
+\c payments
+
+-- grant USAGE and SELECT permissions in the 'public' schema
+GRANT USAGE ON SCHEMA public TO "db-o11y";
+GRANT SELECT ON ALL TABLES IN SCHEMA public TO "db-o11y";
+
+-- grant USAGE and SELECT permissions in the 'tests' schema
+GRANT USAGE ON SCHEMA tests TO "db-o11y";
+GRANT SELECT ON ALL TABLES IN SCHEMA tests TO "db-o11y";
+```
+
+Alternatively, use the predefined role `pg_read_all_data` to grant `USAGE` and `SELECT` permissions to all objects at once:
+
+```sql
+GRANT pg_read_all_data TO "db-o11y";
+```
+
 ### Running and configuring Alloy
 
 1. You need to run the latest Alloy version from the `main` branch. The latest tags are available here on [Docker Hub](https://hub.docker.com/r/grafana/alloy-dev/tags) (for example, `grafana/alloy-dev:v1.10.0-devel-630bcbb` or more recent) . Additionally, the `--stability.level=experimental` CLI flag is necessary for running the `database_observability` component.