[ API ] QAN for Postgresql attempts to connect to a database with the same name as the username

Suggested implementation:
1. Modify API in pmm repo for qan endpoint.
2. Add property database in endpoint GetReport in qan-api2.
3. Get right database name for row.
4. Send it to frontend.

How to test (without FE part):

1. Open QAN in grafana
2. Pick some row from postresql.
3. See in debugger endpoint /GetReport where rows contains property datababase. See screenshot:
   

 Impact on the user:

• User is unable to see Table and indexes data/information for PG monitoring

Steps to reproduce:

Verified with PMM 2.13.0 and PG-12

1. Install PMM-server 
2. configure PG  and create a user as described in the document 

https://www.percona.com/doc/percona-monitoring-and-management/2.x/setting-up/client/postgresql.html

Add Postgresql to pmm monitoring:  (Username is pmm_user  )

# pmm-admin add postgresql pgsql --username=pmm_user --password=Admin123 127.0.0.1:5432
PostgreSQL Service added.
Service ID : /service_id/a783e239-70c1-4780-941b-3d5c6e0d0373
Service name: pgsql

PG commands:

postgres=# CREATE DATABASE dbmonitoring;
CREATE DATABASE
postgres-# \l
 List of databases
 Name | Owner | Encoding | Collate | Ctype | Access privileges 
--------------+----------+----------+---------+-------+-----------------------
 dbmonitoring | postgres | UTF8 | en_IN | en_IN | 
 postgres | postgres | UTF8 | en_IN | en_IN | 
 template0 | postgres | UTF8 | en_IN | en_IN | =c/postgres +
 | | | | | postgres=CTc/postgres
 template1 | postgres | UTF8 | en_IN | en_IN | =c/postgres +
 | | | | | postgres=CTc/postgres
(4 rows)
postgres-# \c dbmonitoring
psql (13.1 (Ubuntu 13.1-1.pgdg18.04+1), server 12.5 (Ubuntu 2:12.5-2.bionic))
You are now connected to database "dbmonitoring" as user "postgres".

dbmonitoring=# CREATE TABLE links (
id SERIAL PRIMARY KEY,
url VARCHAR(255) NOT NULL,
name VARCHAR(255) NOT NULL,
description VARCHAR (255),
 last_update DATE
);
CREATE TABLE
dbmonitoring=# INSERT INTO links (url, name)
dbmonitoring-# VALUES('https://www.postgresqltutorial.com','PostgreSQL Tutorial');
INSERT 0 1
dbmonitoring=#

Actual result:
PMM QAN: 

PG Queries (INSERT) has the following in Tables and Examples:

pq: database "pmm_user" does not exist

Expected Result:

• PMM uses the correct database and information about Tables and indexes exists

Workaround:
N/A

Details

Original report:

When using QAN with postgres, click on "tables" tab shows the same error:

Maybe it should connect to the default "postgres" or the public schema instead of attempting 

Suggested implementation:
1. Currently we get table info from example, but it doesn't have database info, so it this field could be added it could be use on a frontend side in  request "v1/management/Actions/StartPostgreSQLShowCreateTable".