Feature request for metrics collection

Title: Metric collection

Proposer: Canonical

Target MDG/TF: SO, VCA

Type: Feature

Description:

Operational decisions are best made when the operator is fully informed. To this end, we are proposing two different sets of metric collection we'd like to see implemented:

Application-level metrics, collected via Juju and the VNF. These kind of metrics are defined by the VNF and collected automatically by the VCA.
Unit-level metrics collected by an agent like collectd or statsd. These are standard metrics that apply to the machine where the VNF/application is running, i.e., the guest OS, not a VM image. This depends on the acceptance of the Full Juju/Charm integration feature request.

To accomplish this, we would like to see the following done:

Explore options for a suitable collection agent, like collectd or statsd, and develop a charm, if none exists, for the chosen collector.
Add common code to a VNF layer to ease adoption of metric collection
Add metric collection to example VNF(s)
Document metric collection wrt VNFs in the wiki

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