Feature Request: Visualize Time Difference Across Spans (End–Start)

### Problem Statement

We’ve recently created a new dashboard for one requirement and we are adding custom spans to our traces (for example, spans A, B, and C) and are using the dashboard to visualize their performance. Currently, the dashboard allows us to visualize combined spans (like the total time for A+B) by using addition. However, this approach also includes non-instrumented gaps between the spans, which makes it difficult to measure the precise interval we care about.

Our requirement is to visualize the difference between two spans, specifically something like:
B’s end time – A’s start time
This would allow us to see the total duration covering a specific interval of interest across spans, rather than just aggregated or overlapping values.
Would it be possible to add this feature to the dashboard, it would be very useful instead of creating separate spans for such use cases.

### Solution Brainstorm

_No response_

### Product Area

Dashboards

Start by reviewing the dashboard’s existing span-combination behavior, especially how addition is represented and evaluated. Define how a query should select the start of one span and the end of another, including the expected result for spans A and B. Done means the dashboard can visualize B’s end time minus A’s start time without requiring separate instrumentation.

Written by the indexing model from the issue text.