Add templating to metricshub.yaml to generate the list of resources to monitor dynamically

[bertysentry]

Idea

Use Velocity for templating in metricshub.yaml.

Use Case

Generate the list of resources to monitor by MetricsHub dynamically from other sources (files, HTTP, scripts, etc.). Integrate with CMDB, etc.

How it would work

Configuration file would be metricshub.yaml.vm in this scenario. User would write YAML mixed with Velocity templating code. In Velocity, the user can use directives (#if, #foreach, etc.), variables and objects ($var), call object methods ($obj.myMethod(arg)). See the Velocity User Guide, Velocity Developer Guide,

User can also call tools exposed by Velocity, as well as our own tools (see how we created the Sentry Maven Skin Tools.

Example

resources:

#set($hostList = $json.fetch("https://my-cmdb/hostList")
#foreach ($host in $hostList.iterator() )
  host:
    attributes:
      host.name: $host.hostname
    protocols:
#if ($host.snmp == "true")
      snmp:
        community: public
#else
      http:
        username: op
        password: op
#end
#end

Ideally, we would create our own $http Velocity tool, with more capabilities than the simple fetch() method of $json.