agents: move text metrics calculation to common
The `Resource` is now updated in `fill_proc_metrics()` so there's no
need for every agent to do it.
agents: make metrics name format configure
So they can be either camelCase or snake_case. Out OTLPAgent uses
`snake_case`. The upcoming GRPCAgent will use `camelCase`.
Also, fix the name of the `gc_dur_us` metric.