Awkward repo name, since elexir projects doing almost the same thing already exists. Add it this way to your rebar.config dependencies (hex release might come later)
{prometheus_hackney_collector, {git, "https://github.com/kivra/prometheus_hackney_collecterl.git", {branch, "main"}}}
Make sure your application depends on hackney and prometheus in your .app file. Then use your application start callback to make prometheus know about this collector when dependencies are running.
prometheus_registry:register_collector(prometheus_hackney_collector)
Each hackney pool will expose http_client_pool_size, http_client_pool_in_use_count,
http_client_pool_free_count, and http_client_pool_queue_count. These are the values returned by
hackney_pool:get_stats(PoolName). To find out the pools in existance it uses a hackney internal ets
table, which is something that could break in future updates to hackney.