mongo::MetricTree is used to gather ServerStatusMetricFields into a tree structure where only leaves represent metrics. They are later plotted in a BSON structure and sent over the wire.

Internally they are represented as two std::map where the key is a std::string; Leaves are exclusively persisted at the _metrics _ map while branches are represented as entirely new _MetricTree in the _subtrees map.

While this is correct, it is memory inefficient.

Acceptance Criteria: provide a better representation to MetricTree internals avoiding the usage of std::map