Add useful Operator metrics

Since #307 we now have generic `go` metrics, like mem, gc, threads etc.

Let's add application level metrics for the operator iself, that could be useful for Grafana Board and alerts. Suggestions:
* Gauge of nuber of currently managed CRD instances for SolrClouds, SolrBackups, SolrPrometheusExporter
* Gauge for CRDs currently in a failure state
* Reconcile stats
* Successful vs failed reconcile events, broken down to what kind of event
* Size of pending operations in reconcile queue (if such a thing)
* Operation stats
* For each operation type (install, upgrade, delete, backup etc) counts and status

Goal would be to make a simple Grafana board where you can filter on namespace etc to see raw operator health, and at a glance whether some operations are in failure state etc. Futher filter by labels like SolrCloud name, so you can see number of failed operations towards each cluster, and when they happened.

Start by reviewing the generic Go metrics added in #307 and the operator's CRD reconciliation and operation paths. Define the metric scope and labels needed for namespace, SolrCloud, and operation filtering, then verify that the resulting metrics support a Grafana board showing managed resources, failures, reconcile outcomes, and operation status.

Written by the indexing model from the issue text.