No version when default metrics disabled

[samsk]

Quick fix, to disable collection of static_version if default metrics are disabled.
It throws an error, when connected to multiple databases on same server:

# curl localhost:19201/metrics
An error has occurred while serving metrics:

collected metric "pg_static" { label:<name:"server" value:"172.16.110.20:5432" > label:<name:"short_version" value:"11.2.0" > label:<name:"version" value:"PostgreSQL 11.2 on x86_64-pc-linux-gnu, compiled by gcc (GCC) 4.8.5 20150623 (Red Hat 4.8.5-36), 64-bit" > untyped:<value:1 > } was collected before with the same name and label values

[coveralls]

Coverage increased (+0.09%) to 61.932% when pulling a2455bb on samsk:master into c9dc7a2 on wrouesnel:master.