There's some concern about the current implementation, as raised in SERVER-119903.

One to make the implementation cleaner is to use a passkey with a name that makes it very clear that it shouldn't be used (e.g. MetricName::internalOnlyDisaggPassKey()) so that new uses would be a big red flag.

Another way is to use macros to make it so calling a different macro only works in certain files, something like

#define MONGO_METRIC_NAME_PASSKEY_NANDO_TEAM_USE_ONLY_
DEFINE_MONGO_METRIC_NAME(disagg_foo)
DEFINE_MONGO_METRIC_NAME(disagg_bar)
DEFINE_MONGO_METRIC_NAME(disagg_bat)
#undef MONGO_METRIC_NAME_PASSKEY_NANDO_TEAM_USE_ONLY_