[MLflow] Add cost aggregation and visualization by trace metadata fields

<!-- issue-warning --> > [!WARNING] > Before submitting a PR, please make sure that: > - A maintainer has triaged this issue and applied the `ready` label > - This issue has no assignee > - No duplicate PR exists > > PRs not meeting these requirements may be automatically closed. ### Willingness to contribute Yes. I can contribute this feature independently. ### Proposal Summary MLflow currently supports associating traces with metadata such as `mlflow.trace.user` and `mlflow.trace.session`. However, MLflow does not yet provide built-in aggregation and visualization of cost by these, and potentially other, metadata fields. This proposal adds native support for cost aggregation by trace metadata dimensions, enabling users to analyze cost by: session, user, agent (see #21586) or other metadata fields. The feature should include UI support for viewing aggregated costs grouped by these fields, similar to how MLflow currently displays aggregated trace-level costs. ### Motivation > ###