Submit k_state.nodes.by_condition as gauge instead of counter

[xvello]

What does this PR do?

Because AgentCheck.count() will submit the resulting count as a counter instead of a gauge, this PR moves to an internal use of a collections.Counter(), to submit node counts as gauge.

Motivation

Ensure the metric type is correct. Upgrading all agents will be necessary for the correct type to be consistently picked up.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
  - [ ] If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo

Additional Notes

Anything else we should know when reviewing?

[xvello]

On a three-node cluster:

    {
      "metric": "kubernetes_state.nodes.by_condition",
      "points": [
        [
          1530634931,
          0
        ]
      ],
      "tags": [
        "condition:DiskPressure",
        "status:unknown"
      ],
      "host": "gke-xvello-110-default-pool-ee890f07-1s1k.c.datadog-sandbox.internal",
      "type": "gauge",
      "interval": 0,
      "source_type_name": "System"
    },
    },
    {
      "metric": "kubernetes_state.nodes.by_condition",
      "points": [
        [
          1530634931,
          0
        ]
      ],
      "tags": [
        "condition:DiskPressure",
        "status:true"
      ],
      "host": "gke-xvello-110-default-pool-ee890f07-1s1k.c.datadog-sandbox.internal",
      "type": "gauge",
      "interval": 0,
      "source_type_name": "System"
    },
    {
      "metric": "kubernetes_state.nodes.by_condition",
      "points": [
        [
          1530634931,
          3
        ]
      ],
      "tags": [
        "condition:DiskPressure",
        "status:false"
      ],
      "host": "gke-xvello-110-default-pool-ee890f07-1s1k.c.datadog-sandbox.internal",
      "type": "gauge",
      "interval": 0,
      "source_type_name": "System"
    },

[hkaj]

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