Overview

The MetricTreeSet (per-ClusterRole metric trees) was collapsed into a single global MetricTree. As a result, AppendMergedTreesInvocation in src/mongo/db/commands/server_status/server_status_metric.cpp — a k-way merge algorithm over std::vector — is now only ever invoked with exactly one tree (from MetricTree::appendTo).

Scope of Work

* Replace AppendMergedTreesInvocation with a simple single-tree recursive walker that preserves the current output contract: two-phase traversal (leaf metrics before subtrees) and support for excludePaths.
* Remove the merge-only machinery: min-key cursor selection across cursors, the collision uassert, the dead _trees member.
* The merge-specific unit tests (e.g. MergedTreeView) were already removed in the collapsing change; add/keep tests covering exclusion and serialization order for the single tree.

Notes

Identified during review of the remove-scoped-role-metrics refactor. Purely a simplification — no behavior change intended.