Low-overhead counter and latency-sample store for SuperCache observability.

All state is held in a single :public ETS table owned by this GenServer. Writes use update_counter/3 (atomic) and are safe to call from any process without coordination overhead.

Counter keys

Counters are stored as {namespace, field} tuples:

{:tpc, :committed}
{:tpc, :aborted}
{:api, :put}, :calls}          # note: nested via get_all/1
{{:api, :put}, :errors}

Latency samples

Latency ring buffers are stored as {:latency, key} records holding a list of the most recent @max_samples microsecond values. The list is bounded so memory usage is constant regardless of call volume.

Usage

# In Router / Replicator — after an operation completes:
Metrics.increment({:api, :put}, :calls)
Metrics.push_latency({:api_latency_us, :put}, elapsed_us)

# Reading back:
Metrics.get_all({:api, :put})
# => %{calls: 1_200, errors: 2}

Metrics.get_latency_samples({:api_latency_us, :put})
# => [45, 110, 88, ...]