…s already one in main

implement histogram and gauge. make existing counter conform to standard. rename old "gauge" to "gauge_fn" since it is not a true prometheus gauge but an equivalent of the golang GaugeFunc

metrics structs can use normal initialization so they aren's so esoteric. the registry is the context where we'd like to allocate everything in an arena and manage the memory centrally. so, for clarity, that scope can also be in charge of allocating the structs. that way, the structs can behave as normal structs on their own, without requiring special memory management

…am buckets ownership

- initialize prometheus registry and http adapter with sig main()
- add global registry singleton.
- implement OnceCell to safely support global singletons that need to be initialized at runtime
- make Counter a normal looking struct (not a file struct)
- switch unnecessary .Release operation in histogram to .Monotonic
- switch from std.http to httpz