MINOR: contrib/prometheus-exporter: Rename some metrics to be more usable

MINOR: contrib/prometheus-exporter: Rename some metrics to be more usable

Some metrics have been renamed and their type adapted to be more usable in
Prometheus:

  * haproxy_process_uptime_seconds -> haproxy_process_start_time_seconds
  * haproxy_process_max_memory -> haproxy_process_max_memory_bytes
  * haproxy_process_pool_allocated_total -> haproxy_process_pool_allocated_bytes
  * haproxy_process_pool_used_total -> haproxy_process_pool_used_bytes
  * haproxy_process_ssl_cache_lookups -> haproxy_process_ssl_cache_lookups_total
  * haproxy_process_ssl_cache_misses -> haproxy_process_ssl_cache_misses_total

No backport needed. See issue #81 on github.