pkg/metrics/mem: remove redundant map write-backs and optimize proxy lookup (#5221)

Remove 4 redundant pointer map write-backs in OpenConnection,
CloseConnection, AddTrafficIn, and AddTrafficOut since the map stores
pointers and mutations are already visible without reassignment.

Optimize GetProxiesByTypeAndName from O(n) full map scan to O(1) direct
map lookup by proxy name.