Rework statistic collection to be per repository and add metric output option

Rework statistic collection to be per repository and add metric output option

Many statistic values are now accounted by repository via repo_stat_inc()
At end of the run sum_stats() accumulates these stats per TAL and globally.
The new output file metrics is written when the -m output flag is specified.
The metrics file is written in OpenMetrics format (with a few tweaks to
allow node_exporter to parse the file as well). The ometric code is a copy
from bgpctl(8) and should be kept in sync.
OK tb@