Improve explanation about distributed counting
Improve explanation about distributed counting

- Link to helpful alternatives.

- Acknowledge the need and mention that the Prometheus project might
  offer its own solution one day.

Signed-off-by: beorn7 <91e32ae1d26bd312b370cf8378a5a83accca90a0@soundcloud.com>

prometheus/pushgateway,prometheus/pushgateway,prometheus/pushgateway