auto-discover-databases breaks extend.query-path conflicts

[Drugoy]

If auto-discover-databases is enabled - most queries specified in queries.yaml will result into
An error has occurred while serving metrics: collected metric "pg_slow_queries" { label:<name:"server" value:"localhost:5432" > gauge:<value:0 > } was collected before with the same name and label values.

Solution: auto-discover-databases should add label dbname="your_database" for each metric from queries.yaml.

[ktosiek]

Looks like this got fixed in #334 (released as 0.8) - you can mark the system-level queries with "master: true", and then use a custom LABEL for the database-specific ones.

There's an example in query.yml: https://github.com/wrouesnel/postgres_exporter/blob/e2df41f43effdfbb4544b3efb70350380ad895fe/queries.yaml#L9-L22