Adding Prometheus serialization format

[rodoufu]

It should add an option to fix #51

Currently metrics are generated using the OpenMetrics format https://github.com/prometheus/OpenMetrics/blob/main/specification/OpenMetrics.md#counter-1

# HELP my_counter_seconds My counter.
# TYPE my_counter_seconds counter
# UNIT my_counter_seconds seconds
my_counter_seconds_total 0
# EOF

This MR adds the ability to generate then in the Prometheus format as well.
The prometheus format does not add the _total suffix to counter metrics neither the _info to info metrics.

# HELP my_counter_seconds My counter.
# TYPE my_counter_seconds counter
# UNIT my_counter_seconds seconds
my_counter_seconds 0
# EOF