View transaction tags at span level

### Problem Statement

In the trace explorer, there is currently no way to surface tags such as `browser` at the span level. This prevents us from being able to see span metrics by browser type, device class, etc without manual instrumentation.

### Solution Brainstorm

Add `root` property to span that contains all parent transaction tag values. We could then aggregate on `root.browser` at the span level for example.

### Product Area

Performance

Start by tracing how the trace explorer represents span and parent transaction tags; the issue names no files, tests, or entry points. Define how a span-level root property should expose parent transaction tags, then verify that aggregations such as root.browser work for browser and device-class values.

Written by the indexing model from the issue text.