Provide configurable Prometheus metrics exporters, counting defined search queries

[thjaeckle]

Idea:
It is often useful to provide certain statistics about the amount of "Things" managed within Ditto, for example:

• displaying the total amount of currently Things being in a "ERROR" state
• displaying the total amount of provisioned Things
• displaying the total of Things which were not yet "connected"
• ...

In order to provide this kind of information e.g. to display on a Grafana dashboard, an idea would be to provide those as DittoMetrics.
This would provide metrics complementing Ditto's exported metrics for operating Ditto - on the same Prometheus endpoint to be scraped by Prometheus - directly from the "things-search" service in Ditto.

The idea is that a Ditto operator/admin can then configure "searches" in e.g. the search configuration search.conf, e.g. providing a label.
Rough idea:

operator-metrics {

  scrape-interval = 15m

  produced_cars = {
    namespaces = [
      "io.acme.corp.cars"
    ]
    rql = "exists(attributes/manufactured)"
  }
  errors_in_lamps = {
    scrape-interval = 1m
    namespaces = [
      "io.acme.corp.bulbs",
      "io.acme.corp.leds"
    ]
    rql = "and(eq(attributes/type,'LIGHT_BULB'),in(features/health/properties/state,'CRITICAL','ERROR'))"
  }
}