GleanPingView: Move all Glean internal metrics to a single category

[fbertsch]

We currently have several Glean internal metrics represented in the Glean Ping categories, mainly in the metrics ping. These include: glean.error, glean.validation, glean.database, etc.

We should move all of these to a single glean category. The remaining part of the actual category should be prepended to the dimension name. For example, we currently have:

dimension: metrics__memory_distribution__glean_database_size__sum {
    sql: ${TABLE}.metrics.memory_distribution.glean_database_size.sum ;;
    type: number
    group_label: "Glean Database"
    group_item_label: "Size Sum"
}

We would drop Database from the group_label, and prepend it to the group_item_label (We can also consider a separator to make it clear what is part of the category and what is the metric):

dimension: metrics__memory_distribution__glean_database_size__sum {
    sql: ${TABLE}.metrics.memory_distribution.glean_database_size.sum ;;
    type: number
    group_label: "Glean"
    group_item_label: "Database - Size Sum"
}