Add calculate_rate for NetworkIn/Out, DiskRead/WriteBytes and DiskRead/WriteOps

[kaiyan-sheng]

This PR is to try solve elastic/kibana#42687 in metricbeat side.
In ec2 metricset, adding a config parameter called calculate_rate to calculate rates for 8 metrics:

network.in.bytes_per_sec = network.in.bytes / 300 or network.in.bytes / 60
network.out.bytes_per_sec = network.out.bytes / 300 or network.out.bytes / 60
network.in.packets_per_sec = network.in.packets / 300 or network.in.bytes / 60
network.out.packets_per_sec = network.out.packets / 300 or network.out.bytes / 60
diskio.read.bytes_per_sec = diskio.read.bytes / 300 or diskio.read.bytes / 60
diskio.write.bytes_per_sec = diskio.write.bytes / 300 or diskio.write.bytes / 60
diskio.read.ops_per_sec = diskio.read.ops / 300 or diskio.read.ops / 60
diskio.write.ops_per_sec = diskio.write.ops / 300 or diskio.write.ops / 60

Right now both the original metrics and the rate metrics are reported by ec2 metricset, in the future, the original metrics will be removed.

[kaiyan-sheng]

If detailed monitoring is enabled, then these metrics will be sent to Cloudwatch every 1 min instead of every 5min. The rate calculated here is for each data point, which has nothing to do with the collection period that user sets in Metricbeat aws module. Therefore this calculation doesn't seem to be able to replace by adding collection period into each event. @jsoriano What do you think?

[jsoriano]

Added some comments, and this will also need a changelog entry.

[jsoriano]

It LGTM, but it needs a changelog entry.