MongoDB exprorter doesn't show version for mongos intstance

Steps to reproduce:

1. Run Shard with mongos instance (you can use this docker compose file https://github.com/percona/mongodb_exporter/blob/main/docker-compose.yml#L156 to run it locally)
2. Run mongodb_exporter and connect it to mongos instance (For example

   ./mongodb_exporter --mongodb.uri=mongodb://localhost:17000 --collector.replicasetstatus=true --log.level=debug --collect-all --web.listen-address=:9219 --compatible-mode=true
   

   )

3. open metrics page (in this case http://localhost:9219/metrics)
4. find metric `mongodb_version_info`

Expected result
In this case in the label `mongodb` version you can see mongo version (for example

mongodb_version_info{mongodb="4.2.23"} 1

)

Actual result
You can see this:

mongodb_version_info{mongodb="server version is unavailable"} 1