InitPrometheus initialises the Metrics struct with the various metrics and registers the metrics with a *prometheus.Registry.

NewESCollectorFunc creates a function that takes an *elasticsearch.Client and returns a prometheus.Collector, the namespace and subsystem for the prometheus metrics must be provided at creation time.

ESCollector collects the metrics from the ES client so that we can serve them with the rest of the metrics.

Metrics struct contains the various prometheus metrics collected by the indexer.