Enable script processor for Metricbeat

[exekias]

This PR enables the script processor in Metricbeat. We can use it for some data conversion in events coming from Lightweight modules.

[exekias]

@andrewkroh pinging you just in case there is some good reason not to do this. I see initially this was done mainly for Filebeat

[mtojek]

LGTM based on references to other beats. Nice!

[jsoriano]

We can use it for some data conversion in events coming from Lightweight modules.

Take into account that light modules don't support processors at the moment.

[exekias]

yes, I noticed it, as of now the workaround would be to add the processor in the config.yml file, which seems to work.

Any plans to add support at the manifest level? that would be cool

[mtojek]

I introduced these changes into fresh ActiveMQ module and it seems to report correctly:

  "activemq": {
    "broker": {
      "name": "localhost",
      "producers": {
        "count": 0
      },
      "mbean": "org.apache.activemq:brokerName=localhost,type=Broker",
      "memory": {
        "broker": {
          "pct": 0.18
        },
        "store": {
          "pct": 0
        },
        "temp": {
          "pct": 0
        }
      },
      "consumers": {
        "count": 0
      },
      "messages": {
        "count": 115170,
        "enqueue": {
          "count": 119854
        },
        "dequeue": {
          "count": 1249

[jsoriano]

yes, I noticed it, as of now the workaround would be to add the processor in the config.yml file, which seems to work.

Yep, it would work in the config, but distributing logic this way can be error-prone, for example on upgrades the script wouldn't get updated if it is in the config.

Any plans to add support at the manifest level? that would be cool

Yes, this would increase a lot the possibilities of light modules, specially having the script processor. There are no plans for this yet, we could increase priority of that if we consider it valuable.

From what I remember from implementing light modules, the problem with adding processors support can be that they are configured with the pipeline before the module is loaded, but with light modules we wouldn't know what processors to add till we load the module. So we will need some mechanism to add processors to the pipeline during module instantiation, or a way to add a "pipe" to process events before sending them to the pipeline.

I will create an issue for that.

[jsoriano]

Added issue for processors in light modules #14740