sql exporter is failing to retrieve the number of running queries

The sql exporter is complaining about the query executed to get the number of queries running on the database :

Nov 30 16:56:48 belvedere.internal.softwareheritage.org prometheus-sql-exporter[3449201]: {"caller":"level.go:63","db":"postgres","err":"Column 'usename' must be type text (string)","host":":5433","job":"12/main","level":"error","msg":"Failed to update metric","query":"running_queries","ts":"2020-11-30T16:56:48.217844289Z","value":"count"}
Nov 30 16:56:48 belvedere.internal.softwareheritage.org prometheus-sql-exporter[3449201]: {"caller":"level.go:63","db":"postgres","err":"zero values found","host":":5433","job":"12/main","level":"error","msg":"Failed to update metrics","query":"running_queries","ts":"2020-11-30T16:56:48.217915131Z"}

(the same logs are also present on other database servers)

[discovered during #2828 (closed)]

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Migrated from T2831 (view on Phabricator)