Allow filtering crash-free sessions/users by custom event tags

### Problem Statement

Currently, crash-free sessions and users are calculated from Release Health data, but there’s no option to filter these metrics by custom event tags. This would be useful for comparing stability between different tagged populations (e.g., feature configurations) without having to manually calculate percentages using external tools.

### Solution Brainstorm

Enable filtering crash-free metrics by any event tag within the Sentry UI, including dashboards and Release Health views.

### Product Area

Releases

The issue names no files, tests, or entry points. Start by tracing the Release Health crash-free sessions/users views and dashboard filters; define the tag-filtering behavior and verify it across both surfaces.

Written by the indexing model from the issue text.