Make Feature Flags Searchable in Traces View & Transactions view in Discover

### Problem Statement

Feature flags are captured and visible in Issues, but they cannot be searched or filtered in Traces view or Transactions in Discover.

For teams with many flags (50+), this limits their ability to debug performance regressions tied to specific rollouts.

### Solution Brainstorm

- Extend feature flag search support to Traces view and Transactions in Discover.
- Align search behavior across Issues, Explore, and Traces, so feature flags work consistently.
- Enable filtering by flags[flag-name] similar to tags.

### Product Area

Performance