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