add a metrics endpoint

[nvdk]

Currently certain metrics are only available in the logs while they have a relevance from both a monitoring perspective and (in some cases) a frontend perspective.

The following metrics might be useful:

• length of queue of to be processed delta's
• length of queue of to be processed updates
• elastichsearch index sizes (which can be derived from the elastich search stat api
• amount of errors on ingesting to elasticsearch

perhaps others

We should consider whether we offer these metrics in a json:api format or using open metrics.

[sergiofenoll]

https://openmetrics.org doesn't resolve for me, link should probably be https://openmetrics.io/?