This ticket is to add Stream Expression support to the RollupStream as discussed in ~~SOLR-7560~~.

Proposed expression syntax for the RollupStream (copied from that ticket)

```
rollup(
someStream(....),
over="fieldA, fieldB, fieldC",
min(fieldA),
max(fieldA),
min(fieldB),
mean(fieldD),
sum(fieldC)
)
```

This requires making the *Metric types Expressible but I think that ends up as a good thing. Would make it real easy to support other options on metrics like excluding outliers, for example find the sum of values within 3 standard deviations from the mean could be

sum(fieldC, limit=standardDev(3))

(note, how that particular calculation could be implemented is left as an exercise for the reader, I'm just using it as an example of adding additional options on a relatively simple metric).

Another option example is what to do with null values. For example, in some cases a null should not impact a mean but in others it should. You could express those as

mean(fieldA, replace(null, 0)) // replace null values with 0 thus leading to an impact on the mean mean(fieldA, includeNull="true") // nulls are counted in the denominator but nothing added to numerator mean(fieldA, includeNull="false") // nulls neither counted in denominator nor added to numerator mean(fieldA, replace(null, fieldB), includeNull="true") // if fieldA is null replace it with fieldB, include null fieldB in mean

so on and so forth.