Consider the metrics:
foo{a="a"} 1
foo{a="b"} 1
bar{a="b"} 1
bar{a="c"} 1
The query
sum by (a) (foo) + sum by (a) (bar)
returns
{a="b"} 2
What I would like is a query that will treat the absence of a given set of labels on the other side of the sum as 0. Essentially I need a query with the following result:
{a="a"} 1
{a="b"} 2
{a="c"} 1
Prometheus is lacking outer join functionality. You can see very in-depth description of situation and possible workaround for left outer join from Brian Brazil here.
Based on the same logic full outer join for your example will look like this:
sum by (a) (foo) + sum by (a) (bar) #inner join
or sum by (a) (foo) #with first line ~left outer join
or sum by (a) (bar) #with first line ~right outer join