Document hardware requirements

[Dieterbe]

@woodsaj is most familiar with this

[woodsaj]

For kafka, cassandra based on our own experience we can make the following suggestions

Kafka: minimum 3 nodes. 8core 32GB ram, 1TB SSD disk. With 3-way replication, each node can comfortably handle up to 60k metrics/second.

Cassandra: minimum 3 nodes. 8core 64GB ram, up to 4TB SSD disk.
Storeage needs vary greatly based on retention policy. Assuming a retention of 10s:8d,1m:60d,30m:2y
and 3-way replication each node can support up to 600k active series.

So a 3 node Kafa cluster + a 3 node cassandra cluster could support 1.8million series sent a 10s interval.

For metrictank resources spefically our current numbers are out of date and we now have a much larger pool of instances to analyse to get improved numbers.