Add HELP & TYPE description to each metric

[GromNaN]

Example:

# HELP process_cpu_seconds_total Total user and system CPU time spent in seconds.
# TYPE process_cpu_seconds_total counter
process_cpu_seconds_total 4.09
# HELP process_virtual_memory_bytes Virtual memory size in bytes.
# TYPE process_virtual_memory_bytes gauge
process_virtual_memory_bytes 2.76258816e+08
# HELP node_vmstat_thp_split_pmd /proc/vmstat information field thp_split_pmd.
# TYPE node_vmstat_thp_split_pmd untyped
node_vmstat_thp_split_pmd 1

[GromNaN]

Illustration: https://github.com/popcorn-time-ru/popcorn-ru/blob/52c4c5ec4ee8eba39120c393ea8fbccac465e12e/src/Controller/MetricsController.php