Database instance count on OS overview is incorrect

Impact on the user 
Minimal, but confusing

Steps to reproduce

• Create a new PMM server
• View OS dashboard
• Compare node count to database instance count (top of the dashboard)

Actual result

• Database instance count = 4
• Node count = 1

Expected Result

• Database instance count = node count

Workaround
Edit the dashboard

Details
The database instance count panel on the OS overview dashboard is counting duplicates of the same instance, such that PMM's own PostgreSQL instance appears as 4 items, when there is only 1

count(
mysql_global_status_uptime{node_name=~"...",region=~".*",node_type=~"generic"} or 
mongodb_mongos_instance_uptime_seconds{region=~".*",node_type=~"generic",node_name=~"..."} or 
pg_up{node_name=~"...",region=~".*",node_type=~"generic"} or 
postgresql_up{node_name=~"...",region=~".*",node_type=~"generic"}
)

Removing the `count` shows the issue quite clearly: