By default query will run on all the databases if "runonserver" is not provided.

If user want the query to be run on multiple database versions, use below string.
  runonserver: "9.5, 9.6"

Example yml file as below. ( e.g. below query will run only on database version 9.5 )

pg_replication:
  query: "SELECT EXTRACT(EPOCH FROM (now() - pg_last_xact_replay_timestamp())) as lag"
  master: true
  runonserver: "9.5"
  metrics:
    - lag:
        usage: "GAUGE"
        description: "Replication lag behind master in seconds"

    Instead of having db version string from yml file, user can define the range of
    database server version where query is to be executed.

    If user want to run the query on database version greater than 10.0.0, use below format.
      runonserver: ">=10.0.0"

      Below are the example of db version range user can defined in yml file.
            <=10.1.0
            >=12.1.0
            =11.0.0
            <9.6.0 || >=11.0.0

Only Server type hold that value.