Bunch of warnings: "metric value or labels are not collected, skip"

[lesovsky]

Describe the bug
A lot of messages like "metric value or labels are not collected, skip" in the log.

Steps to reproduce
Add user-defined metrics with "databases" regexp.

Expected behavior
No such messages have to be.

pgSCV startup options

listen_address: 0.0.0.0:9890
disable_collectors:
  - system
services:
  primary:
    service_type: "postgres"
    conninfo: "host=primary port=5432 dbname=postgres user=stats"
  standby:
    service_type: "postgres"
    conninfo: "host=standby port=5432 dbname=postgres user=stats"
collectors:
  postgres/custom:
    subsystems:
      connected_clients:
        databases: "postgres"
        query: |
          SELECT coalesce(host(client_addr), 'local') AS address, usename AS user, datname AS database, count(*) AS total 
          FROM pg_stat_activity
          WHERE backend_type = 'client backend'
          GROUP BY 1,2,3
        metrics:
          - name: total
            usage: GAUGE
            value: total
            labels: [ address, user, database ]
            description: "Number of client connections established to instance per address, user and database."

Errors and Logs

pgscv_1            | {"level":"warn","service":"pgscv","time":"2022-05-08T07:02:29Z","message":"metric value or labels are not collected, skip"}
pgscv_1            | {"level":"warn","service":"pgscv","time":"2022-05-08T07:02:29Z","message":"metric value or labels are not collected, skip"}
pgscv_1            | {"level":"warn","service":"pgscv","time":"2022-05-08T07:02:29Z","message":"metric value or labels are not collected, skip"}
pgscv_1            | {"level":"warn","service":"pgscv","time":"2022-05-08T07:02:29Z","message":"metric value or labels are not collected, skip"}
...

Environment (please complete the following information):

• Used OS (or Containers): [output of cat /etc/os-release]

   NAME="Ubuntu"
   VERSION="20.04.4 LTS (Focal Fossa)"
  

• pgSCV Version [output of pgscv --version]
  lesovsky/pgscv:v0.8.0-beta.4

• PostgreSQL Version [output of psql -c 'select version()']
  PostgreSQL 14.2

• Do PostgreSQL and pgSCV are running on the same host?
  yes

Additional context
none