Trace Explorer - Bring back tag/attribute heatmap

### Problem Statement

Now we are going towards Spans more and more (making Transactions less important - even sunsetting that eventually), are there any conversations about introducing some kind of Spans Attributes/tags heatmap like we have for errors & transactions?

It would really be insightful to figure out what % of the spans show are for example from a certain attribute that they custom setup

### Solution Brainstorm

_No response_

### Product Area

Performance

No file, test, or entry point is named. Start by reviewing the existing error and transaction heatmap implementations and the Trace Explorer span-attribute flow; done means Trace Explorer can show the percentage of spans grouped by a custom attribute or tag.

Written by the indexing model from the issue text.