Metric model interval type

[robert-sanfeliu]

Improve documentation on metric interval type (all, first, last, single, etc...) for raw and composite metrics. For input and output

https://github.com/eu-nebulous/nebulous/wiki/3.1.3-Monitoring-metrics-and-SLOs

[robert-sanfeliu]

in every 30-sec period many events may enter or exit the window (which is a memory buffer)... so calculation will be triggerred "all" means that all calculation will be retained and sent at the end of the 30-sec period "first/last" mean that only the first/last calculation will be retained and sent

[robert-sanfeliu]

for PUSH raw metrics, if interval is defined, and the interval defined is smaller than the frequency the app comunicates readings for the metric, the last known value will be used

[robert-sanfeliu]

http://esper.espertech.com/release-7.0.0/esper-reference/html/

[robert-sanfeliu]

http://esper.espertech.com/release-7.0.0/esper-reference/pdf/esper_reference.pdf