Support Usage Mode in new Trace Explorer to make it easier to track Span Consumption

### Environment

SaaS (https://sentry.io/)

### What are you trying to accomplish?

I have following two questions

## Need clarity on span calculation and how is counted for costing
I am trying to evaluate the overall span usage by different category and operation for specific time period,
Here I am checking for the last 30 days.

As you can see on the traces pages over overall span count look going >9M (by doing sum of all span categories) while when I see it on the Stats dashboard it shows Span usage as 3.5M.

I don't understand how span is calculated here, which span are getting calculated in costing.



## As you can see there are multiple span are getting created with `mark`, `measure`, `ui.long-animation-frame` Is this getting used in any metric evaluation of insights and can I discard this with `tracingSampler` to save a span usage ?


### How are you getting stuck?

Yes, We are planning to introduce Tracing and Performance monitoring on all product lines, but we are not able to calculate the total costing and usage due to this discrepancy

### Where in the product are you?

Performance

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Start by reviewing the new Trace Explorer and the Stats dashboard for the reported 30-day span-count discrepancy. Trace how span categories, operations, and spans such as mark, measure, and ui.long-animation-frame are represented, then define the Usage Mode behavior and verify that costing and sampling guidance answer the reported questions.

Written by the indexing model from the issue text.