Function is_truncation_sentinel_name 

pub fn is_truncation_sentinel_name(name: &str) -> bool

Predicate for “this metric name / map key is the walk-truncation sentinel.” Centralises the literal-equality check so every consumer stays in sync when the sentinel name changes, and so future sentinel variants (e.g. a parser-rejection sentinel) can be threaded through one predicate instead of scattered string literals.

Visibility aligned with walk_json_leaves and WALK_TRUNCATION_SENTINEL_NAME so external consumers have a complete sentinel-discrimination API.