Each mongod and mongos has one global MetricTreeSet via globalMetricTreeSet(), which returns a reference to a process-wide StaticImmortal<MetricTreeSet>.

• MetricTreeSet/MetricTree do not own a lock that serializes every mutator and reader on those trees.
• The global trees are populated by MetricBuilder/ CustomMetricBuilder at file scope (startup). It does not enforce init-only registration.

SERVER-123237 is making MetricsService::create* support registering OTEL metrics to the global MetricTreeSet for serverStatus reporting. Without adding a read/write mutex or some synchronization, OTEL metric creation would only be safe during initialization.