VictoriaMetrics agent does not collect some metrics from PostgreSQL exporter

User Impact:

As a user of PMM Server I'm not able to get data on PostgreSQL / PostgreSQL Instances Overview page for following Dashboards:

• Max Allocated Buffers
• Total Written Files to disk
• Total Files Synchronization to Disk

Steps to reproduce:

• Run PMM Server and PMM Agent
• Run PostgreSQL database instance and add it to monitoring (change username/password and address/port please):
  • pmm-admin add postgresql --username=postgres --password=postgres pg-local 127.0.0.1:5432
• Connect to PostgreSQL exporter web interface (e.g. 127.0.0.1:42002 - please check port on PMM Inventory or by $ pmm-admin list command) and click "Metrics" link
  • Page with metrics should be displayed
• Connect to VM agent web interface (e.g. 127.0.0.1:42002 - please check port on PMM Inventory or by $ pmm-admin list command) and click the "targets" link
  • Page with targets should be displayed
• Find PostgreSQL exporter by agent_id on "targets" page and click the Endpoint link
  • Page with metrics from PostgreSQL exporter, collected by VM agent, should be displayed
• Find following metrics on both pages (VM agent and PostgreSQL exporter):
  • pg_stat_bgwriter_checkpoint_write_time 
  • pg_stat_bgwriter_buffers_alloc
  • pg_stat_bgwriter_checkpoint_sync_time
  • pg_stat_bgwriter_buffers_backend_fsync
  • pg_stat_bgwriter_buffers_backend
  • pg_stat_bgwriter_buffers_clean
  • pg_stat_bgwriter_buffers_checkpoint

Actual result:

Following metrics are present on PostgreSQL exporter metrics page, but not present on VM Agent metrics page for PostgreSQL exporter

Expected result:

Following metrics are present on both pages