Consolidate metrics code in a single package, add metrics prefix config option

[Evedel]

Currently, the Prometheus metrics code is slightly all over the place, which is making it difficult to figure out the code and available metrics.

This PR:

1. proposes to extract all the metrics code into a separate metrics package pkg/metrics/metrics.go
   • no more prom imports and variables all over different packages
   • the metrics package is then dependency injected via input parameter, enabling testing of metrics
2. metricsNamePrefix is added as an optional parameter
   • if empty => all the old metric names are used, so fully backward compatible, but will warn user to set a prefix manually

     k logs event-exporter-678958f5dc-c2wwd
     {"level":"warn","caller":"/app/pkg/exporter/config.go:86","message":"metrics name prefix is empty, setting config.metricsNamePrefix='event_exporter_' is recommended"}
     

   • if set => all the metrics will be prefixed, easy to discover available
     
   • config validation is tested

[Evedel]

While this pr is backward compatible, it seems to me that ideally, metrics are planned for renaming in some future major version bump.

1. events_sent => events_received (it is more accurate description of what this metric is counting and will be consistent with the logging too)
2. all metrics are prefixed by default (as it is a prom standard)
3. all metrics are also suffixed with _total/_count (same prom standard, but for a unit-less accumulating count)

[mustafaakin]

Hello, thanks for the PR, nice change. Metrics definitely look much better now.