Enabling user, group quotas results in error

[mark-tomich]

When enabling --collector.mmrepquota --collector.mmrepquota.quota-types=user in the gpfs_exporter, an error is thrown by the prometheus code because each fileset can have a different quota for each user (or group, for that matter). The result returned by the /metrics endpoint looks something like this:

* [from Gatherer #1] collected metric "gpfs_user_quota_files" { label:<name:"fs" value:"<filesystem>" > label:<name:"user" value:"<user>" > gauge:<value:0 > } was collected before with the same name and label values.