Export a metric with connected agents versions

Option 1

Each kas instance should export an Int64UpDownCounter (https://opentelemetry.io/docs/specs/otel/metrics/api/#updowncounter) that it increments and decrements when an agent connects. Agent version is put into an attribute. It'd be easiest to do it in the agent_configuration module. Conceptually this metric represents a number per version per kas instance.

Option 2

If we want to provide this information as an API, then we'd do it in a different way: in the agent_registrar module. In that case information should be persisted in Redis and eventually expired to clean it up. We can use a hash with versions + a hash per version. GC each of those + use HLEN to get the number of agents per version.

Store agent id and pod id as hash value. That way we can later build an API to get a list of outdated agents+pods (if some agent Pods are outdated).

Proposed solution

Option 2. That way we have both an API and a metric. API can come in handy later if we want to build an admin dashboard. This can be a instance-wide dashboard or group/project dashboard.