Implement metrics for Correspondence use

[martinothamar]

• Number of messages
• Latency to open message by receiver?
• Error rates (creation, delivery)

[danielskovli]

We're currently tracking number of correspondence orders and their immediate status, but nothing else. We may need to implement an event subscription if we want more details than that. I'm unsure of the available events, but at least CorrespondencePublished and CorrespondencePublishFailed are available.

[danielskovli]

Correction, all available events are listed here: https://docs.altinn.studio/correspondence/getting-started/developer-guides/events/#event-subcription

[martinothamar]

Speaking of docs, we should update this: https://docs.altinn.studio/altinn-studio/reference/monitoring/registry/metrics/

[danielskovli]

We're way behind on docs atm, at least regarding Maskinporten and Correspondence. Though we're only currently in RC, so I guess there's no cause for panic 😬

I'll make an issue to update the monitoring docs