Add pg_stat_statements view for non-superuser instructions

[robbiet480]

Needed this to fix errors like * collected metric "pg_stat_statements_blk_write_time_seconds" { label:<name:"datname" value:"postgres" > label:<name:"rolname" value:"postgres_exporter" > label:<name:"server" value:"localhost:5432" > counter:<value:0 > } was collected before with the same name and label values.

[coveralls]

Coverage remained the same at 64.7% when pulling b2f9c7b on robbiet480:patch-1 into e2df41f on wrouesnel:master.

[jkroepke]

👍 Works for me.

[anarcat]

it should be noted that this doesn't work unless pg_stat_statements is loaded, which is not the case by default on Debian buster 10 (psql 11).

[jkroepke]

@anarcat I'm not a maintainer here, but you can just raise a pull request like this to add the note.