Implement mechanisms to enable/disable/register metric collection during runtime

[rschoene]

It'd be great if there was some kind of mechanism for the following case: I use a process monitor on my bash. After 5 minutes I want to add a metric that I did not think of when starting lo2s. Such a mechanism could, for example, use some RPC-interface within lo2s.

Might be over the top, therefore low priority.