Comparison workflows in Trace Explorer: Add useful ways to debug why key tracing based metrics are anomalous

### Problem Statement

Sentry helps users identify problems in their web vital metrics or mobile metrics - but finding what caused that problem requires manually querying a lot of the tracing data to identify which facet of your service (os, release, device, region etc) might be a problem

Then it takes more effort to figure out what changes in your app - what spans are more frequent or anomalous to cause the problem

### Solution Brainstorm

Sentry can solve this problem in 2 ways

One we allow users to choose any anomaly and help them identify the suspect attributes that might be a problem:

We're working on this feature and trying to target a Oct-Jan timeframe for this

For the second part of the problem, we want users to be able to compare aggregated trace waterfalls to easily find which spans in the tree structure are behaving differently

This is something we're tracking to ship later

### Product Area

Explore

No files, tests, or entry points are identified. Start by reviewing the Explore product area and the proposed anomaly-selection and aggregated trace-waterfall comparison flows; define a concrete deliverable and acceptance criteria before implementation.

Written by the indexing model from the issue text.