Exposes metrics for caches. LKB-2594
This exposes a high level namespace, `cache`, that all cache metrics can
be added to - this makes it easier to make library panels for the caches
as I understand it.
To calculate the current cache fill ratio, you could use the following
query:
```
(
cache_inserted_total{cache="node_info"}
- sum (cache_evicted_total{cache="node_info"}) without (cause)
)
/ cache_capacity{cache="node_info"}
```
To calculate the cache hit ratio, you could use the following query:
```
cache_request_total{cache="node_info", outcome="hit"}
/ sum (cache_request_total{cache="node_info"}) without (outcome)
```
