The current code stops the walk after we have found the first relevant
function. However, in expressions with multiple legs, we will then use
the `HistogramStatsIterator` at most once. This change should make
sure we explore all legs.
The added tests make sure we are not using `HistogramStatsIterator`
where we shouldn't (but the opposite can only be seen in a benchmark
or with a more explicit test).
Signed-off-by: beorn7 <beorn@grafana.com>
0fa70e0f6c