docs/metrics.md: explain Prometheus and Grafana
docs/metrics.md: explain Prometheus and Grafana

docs/metrics.md so far explained just the REST API for retrieving current
metrics from a single Scylla node. In this patch, I add basic explanations
on how to use the Prometheus and Grafana tools included in the
"scylla-grafana-monitoring" project.

It is true that technically, what is being explained here doesn't come
with the Scylla project and requires the separate scylla-grafana-monitoring
to be installed as well. Nevertheless, most Scylla developers will need this
knowledge eventually and suprisingly it appears it was never documented
anywhere accessible to newbie developers, and I think metrics.md is the
right place to introduce it.

In fact, I myself wasn't aware until today that Prometheus actually had
its own Web UI on port 9090, and that it is probably more useful for
developers than Grafana is.

Signed-off-by: Nadav Har'El <625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>
Reviewed-by: Botond Denes <a584fa4d75a0a85d721f44fb6e06315a2853b0b6@scylladb.com>
Message-Id: <20190129114214.17786-1-625169824b719df98706d88646d0aae6f368ae4d@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla