Regex support for measurements

Being able to aggregate the same field across multiple measurements would be very useful when working with multiple measurements. For example:

```
var cpu = stream
|from()
.database('mydb')
.retentionPolicy('myrp')
.measurement(/cpu.*/)
```

Would pull all points from any measurement starting with `cpu`.

Search the Kapacitor codebase for the stream.from() pipeline and measurement() handling, then inspect any related tests. Trace how measurement names are selected and determine where regex matching and aggregation across matching measurements belong. Done means a pattern such as /cpu.*/ selects points from every matching measurement without changing exact-name behavior.

Written by the indexing model from the issue text.