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Add prometheus metrics to the cache handler. This just used prometheus, if the metrics middleware does not setup the handler, there is nobody reading these metrics, but they are still reported. Seems the simplest solution while keeping the whole middleware separation in tact.
cache
cache enables a frontend cache.
Syntax
cache [ttl] [zones...]
ttlmax TTL in seconds, if not specified the TTL of the reply (SOA minimum or minimum TTL in the answer section) will be used.zoneszones it should should cache for. If empty the zones from the configuration block are used.
Each element in the cache is cached according to its TTL, for the negative cache the SOA's MinTTL value is used.
A cache mostly makes sense with a middleware that is potentially slow, i.e. a proxy that retrieves answer, or to minimize backend queries for middleware like etcd. Using a cache with the file middleware essentially doubles the memory load with no concealable increase of query speed.
The minimum TTL allowed on resource records is 5 seconds.
Examples
cache 10
Enable caching for all zones, but cap everything to a TTL of 10 seconds.
proxy . 8.8.8.8:53
cache miek.nl
Proxy to Google Public DNS and only cache responses for miek.nl (or below).