Lazily initialize mapped varz counters before the first metric operation.
Lazily initialize mapped varz counters before the first metric operation.

This will remove the counters for unused services and backend_groups, saving an expected >3GB in cloud L2 GFEs.

Also remove the /varz?dev option because the varz page will have much fewer zeros.

Verified on staging experiment GFEs:
Original: http://screen/S82rxFjOD4n
Lazy initialized varz: http://screen/G4CQkMa7rwU

Protected by FLAGS_gfe2_restart_flag_lazy_initialize_mapped_varz.

PiperOrigin-RevId: 339866745
Change-Id: Ie8818f518e24fecf0a31e9198f6ecbdbf893d494

google/quiche,google/quiche,google/quiche,google/quiche