When we are dealing with stacked queries, it may be beneficial to treat some empty values as being zero value.
For instance, say I want the stacked result of all metrics between a certain time window that have a tag of 'foo'.
The problem with the current stacked limitation is that if any of the metrics with label 'foo' is empty during the time, we cannot calculate the stacked result.
This can be due to things such as having an older metric which contains that label but we are no longer actively gathering metrics for. Or having a new metric which contains that label but we are asking for data before anything was gathered.
A query parameter to enable the behaviour that an empty value is zero could be beneficial in this case. This would allow us to perform a stacked query where some of the metrics contain no active values during the time window.