These queries works with PostgreSQL and were tested against Confluence 5.10. You will need to adjust slightly for other databases. As with all queries against a product database, you will need to define a new read-only data source to access the product's database.
Thanks to Shiran Kleiderman for the initial query and question on extending.
Site Labels
select
name as "Label"
, count(name) as "Count"
from CONTENT_LABEL
left join LABEL
on CONTENT_LABEL.labelid = LABEL.labelid
group by name
order by name
Labels for Space
select
name as "Label"
, count(name) as "Count"
from CONTENT_LABEL
left join LABEL
on CONTENT_LABEL.labelid = LABEL.labelid
left join CONTENT
on CONTENT_LABEL.contentid = CONTENT.contentid
left join SPACES
on CONTENT.spaceid = SPACES.spaceid
where spacekey in ('ds')
group by name
order by name
Labels by Space
select
spacename as "Space"
, name as "Label"
, count(name) as Count
from CONTENT_LABEL
left join LABEL
on CONTENT_LABEL.labelid = LABEL.labelid
left join CONTENT
on CONTENT_LABEL.contentid = CONTENT.contentid
left join SPACES
on CONTENT.spaceid = SPACES.spaceid
group by spacename, name
order by spacename, name