Adding an "info" metric

[C-Duv]

Here is an implementation for new wp_info metric to export WordPress code and database versions as talked in #15.

[C-Duv]

I opened this PR as draft because I fail to make it working, all seems good to me but the new metrics is not printed/returned. All others metrics are returned just fine and there is no error in the logs.

I am not really sure how to tell the plugin it should load/enable this metric, I thought adding it to Default_Metrics_Loader was enough.

Also, I want to provide a JSON for updating the Grafana Dashboard (but I'd rather wait we agree on the metric first).

[C-Duv]

In the end, it finally works fine (I obviously had some caching issue).

Here is the JSON for a table Grafana panel to use in the Dashboard:

{
  "id": 36,
  "type": "table",
  "title": "Versions",
  "targets": [
    {
      "refId": "A",
      "editorMode": "code",
      "expr": "topk(1, wp_info{host=\"$host\"})",
      "legendFormat": "__auto",
      "range": false,
      "instant": true,
      "exemplar": false,
      "format": "table"
    }
  ],
  "options": {
    "showHeader": true,
    "footer": {
      "show": false,
      "reducer": [
        "sum"
      ],
      "fields": ""
    }
  },
  "fieldConfig": {
    "defaults": {
      "custom": {
        "align": "auto",
        "displayMode": "auto",
        "inspect": false
      },
      "mappings": [],
      "thresholds": {
        "mode": "absolute",
        "steps": []
      },
      "color": {
        "mode": "thresholds"
      }
    },
    "overrides": []
  },
  "pluginVersion": "9.3.1",
  "transformations": [
    {
      "id": "merge",
      "options": {}
    },
    {
      "id": "organize",
      "options": {
        "excludeByName": {
          "Time": true,
          "job": true,
          "instance": true,
          "host": true,
          "__name__": true,
          "scheme": true,
          "Value": true
        },
        "indexByName": {
          "version": 0,
          "db_version": 1,
          "Time": 2,
          "__name__": 3,
          "host": 4,
          "instance": 5,
          "job": 6,
          "scheme": 7,
          "Value": 8
        },
        "renameByName": {
          "__name__": "",
          "db_version": "Database Version ",
          "version": "WordPress Version "
        }
      }
    }
  ]
}

It adds "Versions" table panel looking as follows:

The PromQL is topk(1, wp_info{host="$host"}) with some transformation to keep only 2 fields, and rename them.

[C-Duv]

I wonder if I should also return the exporter plugin's version?

[Mte90]

I think that is perfect as it is.
About the plugin version I don't think that is too much important as it isn't updated so often as you can see.