[WIP] Add TSDB statistics collector

[simonpasquier]

This is a very naive attempt to enrich metrics usage with statistics.

The PR adds a new TSDB collector which polls the /api/v1/status/tsdb endpoint for the top-N metrics and grabs the number of series broken down by label for the matched metrics.

Configuration example:

tsdb_collectors:
  - enable: true
    period: 1m
    limit: 100
    prometheus_client:
      url: "http://localhost:9090/"

Output example:

{
  "up": {
    "labels": [
      "instance",
      "job"
    ],
    "statistics": {
      "period": 60,
      "series_count": 6,
      "label_value_count_by_label_name": [
        {
          "name": "instance",
          "value": 6
        },
        {
          "name": "job",
          "value": 6
        }
      ]
    },
    "usage": {
      "alertRules": [
        {
          "prom_link": "http://localhost:9090/",
          "group_name": "node",
          "name": "InstancesPerJobDown",
          "expression": "1 - avg by (job) (up) >= 0.5"
        }
      ]
    }
  }
}