Investigate implementing metrics using a single global array

[swlynch99]

Currently, any pelikan server needs to create a large struct containing all the metrics individual systems need in order for the majority of the metrics functionality to work. It would be convenient if new metrics could be registered in a more distributed fashion so that main doesn't need to know about every metrics type.

On the other hand, main and other places - such as the admin server - still need to have a way to see all the metrics.

However, if all the relevant metrics are in rust then it should be possible to use the linkme crate to build a distributed array while still letting individual implementations access metrics as-if they were globals.

[alaymodi]

is this issue still open? I am not too familiar with rust but I think this would be interesting to look into.