Per-server concurrency limiter backed by ETS.

Tracks the number of in-flight requests per base_url using an atomic ETS counter (:ets.update_counter/3). When the limit is reached subsequent requests are rejected immediately — no blocking, no queue.

Configuration

Set max_concurrency per request or globally:

# Limit to 5 concurrent requests (non-blocking)
max_concurrency: 5

# No limit (default)
max_concurrency: :infinity

Behaviour

• The limiter is checked before the HTTP dispatch.
• acquire/2 atomically increments the counter and returns :ok if the slot is available, or {:error, %{code: :max_concurrency_reached}}.
• release/1 decrements the counter after the request completes (success or failure).
• ETS table is created lazily on first use — no supervision tree.