feat: basic stats

[adamalton]

WIP: beginnings of storing basic metrics to be fetched by Prometheus.

Will fix #83 (partially).

Some notes for implementation:

• Number of created records is different to the number of records that exist.
• Counting the number that exist will either involve doing a count() query on the IPNSRecord Durable Object namespace (which might be pretty slow, TBC) or keeping track of creations and deletions.
• We can track creations by using a CF queue; each time a record is created we push a message to the queue, and then those updates get can batched into order to write to a single, global, DO every few seconds to increment the number of created records. This should allow a global counter while not creating contention.
• This creation counter will only increase over time, but Prometheus will regularly fetch it, so the change over time can be handled/analysed by Prometheus.
• Prometheus works by fetching the data every few seconds (we can't push to it), so the w3name CF Worker will need to provide an endpoint to expose to Prometheus giving the latest counter value(s).
• In terms of Prometheus data types, we should use a "counter" for the number of records that have been created (because it only increases). If we want to store the number of records that exist then we should use a "gauge".
• Storing stats for the updates and usage (fetches) can probably work in a similar way to how we count the creations. So I'm thinking that we get the creations counter working first and then we can use the same approach (of batched updates) to tackle the updates and usage metrics.

[adamalton]

This still needs the Cloudflare queues feature to be enabled before we can deploy it, but I think it's now in a good place to have a code review.

Once we've deployed it we'll also need to tell the appropriate people in PL to schedule Prometheus to fetch data from the new /metrics endpoint.

[adamalton]

It's now been suggested that we should use CF Analytics Engine for this instead. So I'm taking this back for some refactoring.