refactor!(libsinsp/metrics): extract metric conversions out of metric…

…s_collector

Separation of concerns: new metrics_converter subclasses

Rename the following metric names for consistency:

cpu_usage_perc_total_host -> host_cpu_usage_perc
memory_used_host -> host_memory_used_kb (or host_memory_used_mb if unit converted)
procs_running_host -> host_procs_running
open_fds_host -> host_open_fds

memory_rss -> memory_rss_kb (or memory_rss_mb if unit converted)
memory_pss -> memory_pss_kb (or memory_pss_mb if unit converted)
memory_vsz -> memory_vsz_kb (or memory_vsz_mb if unit converted)
container_memory_used -> container_memory_used_bytes (or container_memory_used_mb if unit converted)

Signed-off-by: Melissa Kilby <[email protected]>