Update t128_graphql to support compound fields

[gregschrock]

Description

The t128_gaphql telegraf input maps paths from GQL requests to output fields/tags and that mapping is one-to-one. Generally, that is a fair assumption for GQL and we’ve seen other libraries do the same. However, our model has some cases where the data under a request leaf is a tree. We are unable to retrieve such data via the t128_graphql input. We should support a way to get these fields at least as a string JSON blob.

As an example:

    {
      allRouters(name: $routerName) {
        nodes {
          name
          nodes(name: $nodeName) {
            nodes {
              name
              deviceInterfaces(name: $deviceInterface) {
                nodes {
                  extendedStatistics
                }
              }
            }
          }
        }
      }
    }

Produces

{
  "data": {
    "allRouters": {
      "nodes": [
        {
          "name": "burl-corp",
          "nodes": {
            "nodes": [
              {
                "name": "burl-corp-primary",
                "deviceInterfaces": {
                  "nodes": [
                    {
                      "extendedStatistics": {
                        "rx_xoff_packets": 0,
                        "rx_jabber_errors": 0,
                        ...
                      }
                    }
                  ]
                }
              }
            ]
          }
        }
      ]
    }
  }
}

See the readme and unit test for an example of the output format.