Add ::path-cache to registry collectors map metadata

[kgann]

An update to the Java Client Library was merged that improved performance of metric name sanitization. Unfortunately it also includes the move to OpenMetrics that would cause many issues at Nubank. We would like to get the benefit of the metric sanitization performance improvement but cannot upgrade the client at this time.

We shifted our focus to Iapetos in an attempt to implement the improvement here. During the investigation we noticed that collector lookup from the registry sanitizes the metric names on every invocation. In order to limit, as much as possible, the redundant sanitization, we added a simple cache holding the paths to the collectors.

If the cache does not result in a hit, perhaps because the collector was registered with a keyword and then referenced by another supported type such as a vector, the implementation falls back to computing the path again.

For example:

(defonce registry
  (-> (prometheus/collector-registry)
      (prometheus/register
       (prometheus/counter :app/runs-!!!total))))

(.get registry [:app :runs-!!!total] {}) ;; cache MISS, path computed
(.get registry :app/runs-total {}) ;; cache MISS, path computed
(.get registry :app/runs-!!!total {}) ;; cache HIT

Included in the PR is a benchmark of the results:

On the master branch we see:

 {:fn bench/lookup,
  :mode :average,
  :name :registry-lookup,                          ;; metric names do not need to be sanitized
  :score [1.014698602638397 "us/op"]}

 {:fn bench/lookup,
  :mode :average,
  :name :dirty-registry-lookup,                    ;; metric names NEED to be sanitized
  :score [1.9870187159904702 "us/op"]}

With the addition of a cache we observed the following performance improvement:

 {:fn bench/lookup,
  :mode :average,
  :name :registry-lookup,                          ;; metric names do not need to be sanitized
  :score [0.18071412341478607 "us/op"]}

 {:fn bench/lookup,
  :mode :average,
  :name :dirty-registry-lookup,                    ;; metric names NEED to be sanitized
  :score [0.1301492753507884 "us/op"]}