Span Property Math

### Problem Statement

We occasionally have span properties that we would like to perform some metric math on in the context of a single span, rather than across spans.

For example, if I have a span that has both "points" and "factor" properties, it would be great to be able to perform the math `sum(span.points * span.factor)`

The current workaround is to use something like `sum(span.points) * avg(span.factor)`, but this of course produces significantly different results from executing the math on each span before combining.

Ex. with 5 spans that have the following values:
| points | factor |
|--------|--------|
| 1 | 1 |
| 1 | 1 |
| 1 | 1 |
| 1 | 1 |
| 100 | 5 |

`sum(span.points * span.factor)` produces 504, while `sum(span.points) * avg(span.factor)` produces 187.2

### Solution Brainstorm

_No response_

### Product Area

Unknown

No files, tests, or entry points are identified in the issue. Start by locating the span-property aggregation and metric-math implementation, then determine how per-span multiplication could be represented; done means supporting expressions such as sum(span.points * span.factor) with results matching the example.

Written by the indexing model from the issue text.