Add caching for GitHub App tokens

[matheuscscp]

This PR introduces a token cache in the operator. At the moment, we use it for GitHub App tokens in the ResourceSetInputProvider API.

This token cache will report the following metrics:

• Counter flux_token_cache_events_total
  • Help: Total number of cache retrieval events for a Gitops Toolkit resource reconciliation.
  • Labels: event_type, kind, name, exported_namespace. The values of event_type can be cache_miss or cache_hit.
• Counter flux_token_cache_requests_total
  • Help: Total number of cache requests partioned by success or failure.
  • Labels: status. The values of status can be success or failure.
• Counter flux_token_cache_evictions_total
  • Help: Total number of cache evictions.
  • No labels.
• Gauge flux_token_cached_items
  • Help: Total number of items in the cache.
  • No labels.

[stefanprodan]

All metrics exported by Flux Operator use exported_namespace to avoid collision with Prometheus Operator collector where the namespace is set to the pod/service namespace. Can we change that label?

[matheuscscp]

All metrics exported by Flux Operator use exported_namespace to avoid collision with Prometheus Operator collector where the namespace is set to the pod/service namespace. Can we change that label?

Sure, we need a change in the cache lib

[matheuscscp]

All metrics exported by Flux Operator use exported_namespace to avoid collision with Prometheus Operator collector where the namespace is set to the pod/service namespace. Can we change that label?

Fixed, thanks!

[stefanprodan]

LGTM

Thanks @matheuscscp 🏅